Spinal Facet Implants with Mating Articulating Bearing Surface and Methods of Use

ABSTRACT

Superior and/or inferior facets of one or more facet joints may be replaced by superior and/or inferior facet joint prostheses. In one embodiment, a kit of superior or inferior prostheses is provided, in which the prostheses have at least two dimensions that vary among members of the kit independently of each other. Each prosthesis may have a bone engaging surface having a surface that is polyaxially rotatable against a corresponding resection of a vertebra. Each prosthesis may also have an articulating surface shaped such that, after attachment to the spine, the replaced or partially replaced facet joints provide a larger medial-lateral range of motion when the spine is flexed than when the spine is extended. Crosslinks may be used to connect left and right prosthesis together in such a manner that they are stabilized in a position in which they are seated directly against the vertebra.

REFERENCE TO PENDING PRIOR APPLICATIONS

This application is a continuation application of U.S. Ser. No.12/240,070, filed Sep. 29, 2008, which is a divisional of U.S. Ser. No.10/860,543, filed Jun. 2, 2004 by Alan Chervitz et al. for SPINAL FACETIMPLANTS WITH MATING ARTICULATING BEARING SURFACE AND METHODS OF USE(Attorney's Docket No. FSI-03 NPROV); which claims the benefit of (i)U.S. Provisional 60/545,094, filed on Feb. 17, 2004 by Alan Chervitz etal. for SPHERICAL ARTICULATING IMPLANT SURFACE; and (ii) U.S.Provisional 60/545,101, filed on Feb. 17, 2004 by Alan Chervitz et al.for SPHERICAL IMPLANT AND BONE BED. The above-identified documents areherein incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to surgical devices and methods to guideinstruments that prepare the surface of bones and other tissues forimplants that replace a damaged, diseased, or otherwise painful spinalfacet joint.

2. Description of Related Art

Traumatic, inflammatory, metabolic, and degenerative disorders of thespine can produce debilitating pain that can have severe socioeconomicand psychological effects. One of the most common surgical interventionstoday is arthrodesis, or spine fusion, of one or more motion segments,with approximately 300,000 procedures performed annually in the UnitedStates. Clinical success varies considerably, depending upon techniqueand indications, and consideration must be given to the concomitantrisks and complications. For example, Tsantrizos and Nibu have shownthat spine fusion decreases function by limiting the range of motion forpatients in flexion, extension, rotation, and lateral bending.Furthermore, Khoo and Nagata have shown that spine fusion createsincreased stresses and, therefore, accelerated degeneration of adjacentnon-fused motion segments. Additionally, pseudoarthrosis, as a result ofan incomplete or ineffective fusion, may reduce or even eliminate thedesired pain relief for the patient. Finally, the fusion device, whetherartificial or biological, may migrate out of the fusion site.

Recently, several attempts have been made to recreate the naturalbiomechanics of the spine by use of an artificial disc. Artificial discsprovide for articulation between vertebral bodies to recreate the fullrange of motion allowed by the elastic properties of the naturalintervertebral disc that directly connects two opposed vertebral bodies.

However, the artificial discs proposed to date do not fully address themechanics of motion of the spinal column. In addition to theintervertebral disc, posterior elements called the facet joints help tosupport axial, torsional and shear loads that act on the spinal column.Furthermore, the facet joints are diarthroidal joints that provide bothsliding articulation and load transmission features. The effects oftheir absence as a result of facetectomy was observed by Goh to producesignificant decreases in the stiffness of the spinal column in allplanes of motion: flexion and extension, lateral bending, and rotation.Furthermore, contraindications for artificial discs include arthriticfacet joints, absent facet joints, severe facet joint tropism orotherwise deformed facet joints, as noted by Lemaire.

U.S. Pat. No. Re. 36,758 to Fitz discloses an artificial facet jointwhere the inferior facet, the mating superior facet, or both, areresurfaced.

U.S. Pat. No. 6,132,464 to Martin discloses a spinal facet jointprosthesis that is supported on the posterior arch of the vertebra.Extending from this support structure are inferior and/or superiorblades that replace the cartilage at the facet joint. The Martinprosthesis generally preserves existing bony structures and thereforedoes not address pathologies that affect the bone of the facets inaddition to affecting the associated cartilage. Furthermore, the Martininvention requires a mating condition between the prosthesis and theposterior arch (also known as the lamina) that is a thin base of curvedbone that carries all four facets and the spinous process. Since theposterior arch is a very complex and highly variable anatomic surface,it would be very difficult to design a prosthesis that providesreproducible positioning to correctly locate the cartilage-replacingblades for the facet joints.

Another approach to surgical intervention for spinal facets is providedin WO9848717A1 to Villaret. While Villaret teaches the replacement ofspine facets, the replacement is interlocked in a manner to immobilizethe joint.

It would therefore be an improvement in the art to provide a vertebralfacet replacement device and method that provides a relatively highdegree of mobility in the joint, while effectively removing the sourceof arthritic, traumatic, or other disease mediated pain with a minimumof patient discomfort.

SUMMARY OF THE INVENTION

In order to overcome the shortcomings of the prior art, the presentinvention provides a vertebral facet replacement device and method thatreplaces a bony portion of the facets so as to remove the source ofarthritic, traumatic, or other disease mediated pain. Facet jointreplacement in conjunction with artificial disc replacements represent aholistic solution to recreating a fully functional motion segment thatis compromised due to disease or trauma. Together, facet joint and discreplacement can eliminate all sources of pain, return full function andrange of motion, and completely restore the natural biomechanics of thespinal column. Additionally, degenerative or traumatized facet jointsmay be replaced in the absence of disc replacement when the naturalintervertebral disc is unaffected by the disease or trauma.

Accordingly, in certain embodiments, the present invention provides anartificial vertebral facet that replaces the cartilage and a portion ofthe bone of a facet. Furthermore, the invention may provide a method forpreparing a vertebra for the installation of an artificial vertebralfacet, a method for replacing a spinal facet, and possibly, a totalvertebral facet joint replacement.

The present invention may provide numerous advantages over the priorart. One advantage may be that the quality of attachment of theprosthesis is improved. The present invention may provide a precisepress fit into bones, as opposed to relying on prosthetic surfacesmating with highly complex and variable external surfaces of thevertebra, such as the posterior arch or facet. Another advantage may bethat the optional porous coating is placed into interior bone spaceswhere porous coatings have proven to achieve bone ingrowth for excellentlong term fixation strength. This ability to achieve bone ingrowth isuncertain for the prior art devices that engage the external bonesurfaces of the vertebra. Yet another advantage may lie in the removalof the facet bone structure; where the facet bone is involved in thedisease pathology or the trauma that compromised the articular orcartilaginous surface of the facet, resection provides a means forensuring that all pain associated with the disease or trauma is removed.

The above, and other features and advantages of the present invention,will become apparent from the following description, which is to be readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a portion of the spine;

FIG. 2 is a lateral view of a facet joint reconstructed in accordancewith the present invention;

FIG. 3 is a dorsal view of the facet joint shown in FIG. 2;

FIG. 4 is a perspective view of the implanted left inferior facetprosthesis shown in FIGS. 2 and 3;

FIG. 5 is a perspective view of the left inferior facet prosthesis shownin FIGS. 2 and 3;

FIG. 6 is a cranial view of the implanted left superior facet prosthesisshown in FIGS. 2 and 3;

FIG. 7 is a perspective view of the left superior facet prosthesis shownin FIGS. 2 and 3;

FIG. 8 is a perspective view of an alternative implanted left inferiorfacet prosthesis;

FIG. 9 is a perspective view of an alternative left inferior facetprosthesis;

FIG. 10 is a lateral view of an alternative reconstructed facet joint;

FIG. 11 is a dorsal view of an alternative reconstructed facet joint;

FIG. 12 is a perspective view of the implanted left inferior facetprosthesis shown in FIGS. 10 and 11;

FIG. 13 is a perspective view of the alternative left inferior facetprosthesis shown in FIGS. 10 and 11;

FIG. 14 is a cranial view of the alternative implanted left superiorfacet prosthesis shown in FIGS. 10 and 11;

FIG. 15 is a perspective view of the alternative left superior facetprosthesis shown in FIGS. 10 and 11;

FIG. 16 is a perspective view of an alternative bearing surface for thesuperior facet prosthesis shown in FIG. 15;

FIG. 17 is a dorsal view of a single intact vertebra;

FIG. 18 is a lateral view of the same intact vertebra shown in FIG. 17;

FIG. 19 is a dorsal view of the same vertebra of FIG. 17 and FIG. 18,with a portion of the superior facet resected and a portion of theinferior facet resected;

FIG. 20 is a lateral view of the resected vertebra shown in FIG. 19;

FIG. 21 is a dorsal view of the same resected vertebra shown in FIG. 18and FIG. 19 with a fixation element placed through the first superiorresection surface and into the pedicle bone;

FIG. 22 is a dorsal view showing the resected vertebra, the fixationelement, and a superior facet prosthesis;

FIG. 23 is a dorsal view of the vertebra and the implant of FIG. 23 andalso showing the addition of an inferior facet prosthesis;

FIG. 24 is a dorsal view of the implant and vertebra of FIG. 23 and alsoshowing the addition of an enlarged head that has the shape of a lockingnut;

FIG. 25 is a perspective view of a vertebra with an assembled implantcomprising a fixation element, superior facet prosthesis, and a lockingnut;

FIG. 26 is a perspective, cross-sectioned view of the same vertebra andimplant of FIG. 25 with a cross section aligned with the axis of thefixation element;

FIG. 27 is a cranial, cross-sectioned view of the vertebra and implantof FIG. 25, with the section plane positioned as in FIG. 26;

FIG. 28 is a side view of embodiments A, B, C, D, E, and F of thefixation element, a cross-sectional view of each of embodiments A, B, C,D, E, and F, and a side view of the enlarged head in the shape of alocking nut;

FIG. 28A is a side view of embodiments G, H, I, J, K, and L of thefixation element with attached enlarged heads, and a cross-sectionalview of each of embodiments G, H, I, J, K, and L;

FIG. 29 is a perspective view of a radially expanding fixation elementin its unexpanded state;

FIG. 30 is a side view and a bottom view of (i) an expanded radiallyexpanding fixation element and (ii) an unexpanded radially expandingfixation element;

FIG. 31 is a perspective cross-sectional view of a vertebra and a facetimplant showing a cross-pin torsionally and axially securing thefixation element;

FIG. 32 is a dorsal view of a spinal section showing a top, middle, andbottom vertebra with unilateral facet replacements on the right side ofthe spine section, both between the top and middle vertebra, and betweenthe middle and bottom vertebra;

FIG. 33 is a dorsal view of a spine section showing a superiorhemiarthroplasty facet replacement between the top and the middlevertebra and unilateral replacement between the middle and the bottomvertebra;

FIG. 34 is a dorsal view of a spinal section showing an inferior facethemiarthroplasty replacement between the top and the middle vertebra anda unilateral replacement on the right side between the middle and thebottom vertebra;

FIG. 35 is a dorsal view of a spinal section showing a unilateralreplacement between the top and middle vertebrae on the right side, andan inferior facet hemiarthroplasty replacement between the middle andbottom vertebrae on the same side;

FIG. 36 is a dorsal view of a spinal section showing a unilateralreplacement between the top and middle vertebrae on the right side and asuperior facet hemiarthroplasty replacement on the right side betweenthe middle and bottom vertebrae on the same side;

FIG. 37 is a spinal section of two vertebrae showing one inferior facetof the top vertebra and the adjoining superior facet of the bottomvertebra replaced by an articulating facet implant;

FIG. 38 is a perspective view of a curved superior facet prosthesis;

FIG. 39 is a perspective view of a superior facet prosthesis with a boneingrowth surface;

FIG. 40 is a perspective view of an inferior facet prosthesis;

FIG. 41 is a perspective view of an inferior facet prosthesis with abone ingrowth surface;

FIG. 42 is an exploded, perspective view illustrating the addition of alocking washer to the construction of the implant shown in FIG. 25;

FIG. 43 is a perspective view illustrating the implant of FIG. 25 with alocking washer fully installed;

FIG. 44 is a perspective view of the locking washer shown in FIG. 42;

FIG. 45 is a perspective view of superior and inferior facet prosthesesheld against a vertebra by flexible fixation elements;

FIG. 46 is a dorsal view of a bilateral inferior implant;

FIG. 47 is perspective view of a vertebra with an alternative embodimentof a superior facet prosthesis fixed to the bone by one embodiment of afixation element;

FIG. 48 is a perspective, cross-sectional view of the embodiment of thesuperior facet prosthesis and fixation element of FIG. 47 showing thesemispherical shape of the resection and the approximately similarlysemispherical shape of the apposition side of the superior facetprosthesis, as well as an angled resection and corresponding angled flaton the apposition side of the superior facet prosthesis in combinationwith the semispherical resection;

FIG. 49 is a perspective view of the resected vertebra without thesuperior facet prosthesis attached to the vertebra, in which thefixation element is installed in the vertebra;

FIG. 50 is a perspective view of the resected vertebra with the superiorfacet prosthesis attached to the vertebra, with the fixation elementinstalled in the vertebra, but without the locking fastener shown inFIG. 47;

FIG. 51 is a top view of the superior facet prosthesis showing thesemispherical shape of the bone apposition side in combination with theangled flat on the bone apposition side;

FIG. 52 is a rear view of the superior facet prosthesis showing thesemispherical nut engaging surface on the top of the area that is designto connect to the fixation element and the locking nut, or the inferiorprosthesis and the fixation element;

FIG. 53A is a rear view and a perspective view of a plurality ofsuperior facet prostheses of a kit;

FIG. 53B is a top view of an inferior facet prosthesis according to oneembodiment of the invention;

FIG. 53C is a side view of the inferior facet prosthesis of FIG. 53B;

FIG. 53D is a perspective view of a plurality of inferior facetprostheses of a kit;

FIG. 53E is a perspective view showing how a superior facet prosthesisand an inferior facet prosthesis may fit together;

FIG. 53F is a dorsal view of an L5 superior facet prosthesis and an L4inferior facet prosthesis fit on adjacent vertebrae to articulateagainst each other;

FIG. 53G is a posteriolateral view of the implants and vertebrae shownin FIG. 53F;

FIG. 53H is a posteriolateral view showing a cross-section along a firstplane cut through the articulation of the implants of FIG. 53F;

FIG. 53I is a cephalad view showing a cross-section along a second planecut through the articulation of the implants shown in FIG. 53F;

FIG. 54 is a dorsal view of a bilateral inferior facet prosthesis systemand a superior facet prosthesis in situ;

FIG. 55 is a perspective view of the bilateral inferior facet prosthesissystem and the superior facet prosthesis of FIG. 54;

FIG. 56 is a lateral view of the bilateral inferior facet prosthesissystem and superior facet prosthesis in situ;

FIG. 57 is a cranial view of the bilateral inferior implant system insitu;

FIG. 58 is a bottom view of the bilateral inferior facet prosthesissystem in situ;

FIG. 59 is rear view of the bilateral inferior facet prosthesis systemin isolation;

FIG. 60 is a top view of the bilateral inferior facet prosthesis systemin isolation;

FIG. 61 is a bottom view of the bilateral inferior facet prosthesissystem in isolation;

FIG. 62 is a perspective view of the right inferior prosthesis;

FIG. 63 is a perspective view of various ball-shaped members of inferiorprostheses, the ball-shaped members having differing surface features,particularly circumferential grooves, longitudinal grooves, andknurling;

FIG. 64 is an end view of the ball-shaped members of FIG. 63; and

FIG. 65 is a dorsal view of the bilateral inferior facet prosthesissystem, in which castle nuts are attached to the left and right fixationelements.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIG. 1, there is shown a perspective view of a superiorvertebra 1 and an inferior vertebra 3, with an intervertebral disc 2located in between. The superior vertebra 1 has superior facets 43,inferior facets 6, a posterior arch (or lamina) 35 and a spinous process46. The inferior vertebra 3 has superior facets 7, inferior facets 44, aposterior arch (or lamina) 36 and a spinous process 45. Each of thevertebrae 1, 3 also has a pair of pedicles 11.

Referring now to FIG. 2, in a lateral view, the left inferior facet 6 ofthe superior vertebra 1 shown in FIG. 1 has been resected and aninferior facet prosthesis 4 has been attached to the superior vertebra1. Similarly, the left superior facet 7 of the inferior vertebra 3 hasbeen resected and a superior facet prosthesis 5 has been attached to theinferior vertebra 3.

FIG. 3 illustrates a dorsal view of the elements shown in FIG. 2. It canbe appreciated that inferior facet prosthesis 4 replicates the naturalanatomy when compared to the contralateral inferior facet 6 of vertebra1. Similarly, it can be appreciated that superior facet prosthesis 5replicates the natural anatomy when compared to the contralateralsuperior facet 7 of vertebra 3. Neither the inferior facet prosthesis 4nor the superior facet prosthesis 5 rests on the lamina 35.

Turning now to FIG. 4, a perspective view of the superior vertebra 1with implanted inferior facet prosthesis 4 is provided. A bone resectionon the left side of the superior vertebra 1, shown as a resection 31,has removed the natural inferior facet 6 at the bony junction betweenthe inferior facet 6 and the lamina 35. In this manner, any bone painassociated with a disease, such as osteoarthritis, or trauma of the leftinferior facet 6 will be eliminated as the involved bony tissue has beenosteotomized.

FIG. 5 illustrates a perspective view of the inferior facet prosthesis4. A surface 8 replicates the natural articular surface of the replacedinferior facet 6. A post 9 provides a mechanism that can be used toaffix the inferior facet prosthesis 4 to the superior vertebra 1. Thepost 9 is implanted into the interior bone space of the left pedicle 11on the superior vertebra 1 and may or may not extend into the vertebralbody of superior vertebra 1 to provide additional stability.

FIG. 6 illustrates a cranial view of the inferior vertebra 3 with theimplanted superior facet prosthesis 5. A resection surface 32 representsthe bony junction between the natural superior facet 7 and the lamina36.

FIG. 7 illustrates a perspective view of the superior facet prosthesis5. A surface 38 replicates the natural articular surface of the replacedsuperior facet 7. The post 37 provides a mechanism usable to affix thesuperior facet prosthesis 5 to the inferior vertebra 3. The post 37 isimplanted into the interior bone space of the left pedicle 11 (FIG. 6)on the inferior vertebra 3 and may or may not extend into the vertebralbody of the inferior vertebra 3 to provide additional stability.

When the total facet joint is replaced, as shown in FIGS. 2 and 3, thenthe surface 8 (FIG. 5) articulates against the surface 38 (FIG. 7) torecreate the natural biomechanics of the spine motion segment made up ofthe superior vertebra 1, the inferior vertebra 3, and the intervertebraldisc 2. Neither the inferior facet prosthesis 4 nor the superior facetprosthesis 5 rests on the lamina 35 or the lamina 36, respectively.

FIG. 8 illustrates a perspective view of an alternative inferior facetprosthesis 10 that may be implanted into the interior bone space of thelamina 35 of the superior vertebra 1. The interior bone space isaccessed from the resection 31.

FIG. 9 shows a perspective view of the alternative inferior facetprosthesis 10, including a fin 13 that extends into the interior bonespace of the 35. A surface 12 replicates the natural articular surfaceof the replaced facet.

The surfaces of the post 9 (FIG. 5), the post 37 (FIG. 7), and the fin13 (FIG. 9) may or may not include porous coatings to facilitate boneingrowth to enhance the long-term fixation of the implant. Furthermore,such porous coatings may or may not include osteoinductive orosteoconductive substances to further enhance bone remodeling into theporous coating. In this application, the term “implant” refers to anynatural or man-made, fabricated or unfabricated device or group ofdevices that may be added to a human spine. An implant may include oneor more prostheses, one or more fixation devices, and/or othercomponents.

Referring now to FIG. 10, there is shown a lateral view of a superiorvertebra 14 and an inferior vertebra 16, with an intervertebral disc 15located in between. The left inferior facet of the superior vertebra 14has been resected and an inferior facet prosthesis 18 has been attachedto superior vertebra 14 via a screw fastener 17. Similarly, the leftsuperior facet of the inferior vertebra 16 has been resected and asuperior facet prosthesis 19 has been attached to vertebra 16 via ascrew fastener 17.

FIG. 11 illustrates a dorsal view of the elements of FIG. 10. It can beappreciated that inferior facet prosthesis 18 replicates the naturalanatomy when compared to the contralateral inferior facet 22 of thesuperior vertebra 14. Similarly, it can be appreciated that superiorfacet prosthesis 19 replicates the natural anatomy when compared to thecontralateral superior facet 21 of the inferior vertebra 16. Neither theinferior facet prosthesis 18 nor the superior facet prosthesis 19 restson the lamina of the corresponding vertebra 14 or 16.

Turning now to FIG. 12, there is provided a perspective view of thesuperior vertebra 14 with the implanted inferior facet prosthesis 18. Aresection 34 has removed the natural inferior facet at the bony junctionbetween the inferior facet and the adjoining lamina. In this manner, anybone pain associated with a disease, such as osteoarthritis, or traumaof the natural inferior facet 22 will be eliminated inasmuch as theinvolved bony tissue has been osteotomized.

FIG. 13 illustrates a perspective view of the inferior facet prosthesis18. A surface 23 replicates the natural articular surface of thereplaced facet. A flange 25 contacts the pedicle 11 (FIG. 12) and a hole24 receives the screw fastener 17 to attach the inferior facetprosthesis 18 to the superior vertebra 14.

FIG. 14 illustrates a cranial view of the inferior vertebra 16 with theimplanted superior facet prosthesis 19. A resection surface 33represents the bony junction between the natural superior facet 21 (FIG.11) and the corresponding lamina.

FIG. 15 illustrates a perspective view of the superior facet prosthesis19. A surface 27 replicates the natural articular surface of thereplaced facet. A flange 39 contacts the pedicle 11 (FIG. 14) and hole26 receives a screw fastener 17 to attach the superior facet prosthesis19 to the inferior vertebra 16.

FIG. 16 provides a perspective view of an alternative superior facetprosthesis 40 with a bearing surface 41 that mounts to substrate 42. Thebearing surface 41 is a biocompatible polymeric material, such as ultrahigh molecular weight polyethylene. Alternatively, the bearing surfacecan be ceramic, such as zirconia or alumina. The substrate is abiocompatible metal alloy, such as an alloy of titanium, cobalt, and/oriron.

The bearing surface 41 may be formed separately from the remainder ofthe superior facet prosthesis 40, so that the bearing surface 41 and theremainder form components that can be assembled as needed. A kit ofdifferently-sized prostheses may include multiple bearing surfaces likethe bearing surface 41 that may have different thicknesses, articulatingsurface shapes, material selections, and the like. Such a kit may alsoinclude other differently-sized components designed such that somesubset of the components can be selected and assembled together toprovide a prosthesis having the desired dimensions. Prosthesis kits willbe shown and described in greater detail subsequently.

Referring to FIG. 17 and FIG. 18, a single intact vertebra 100 is shown.FIG. 17 is a dorsal view of the vertebra 100. FIG. 18 is a lateral viewof the same vertebra 100. Similar to the two vertebrae 1, 3 shown in theportion of the spine illustrated in FIGS. 1 through 3, the vertebra 100has posterior anatomy comprising left and right superior facets 43 onthe superior, or top side in this view of the dorsal vertebra 100, leftand right inferior facets 6 on the inferior or bottom side of theposterior vertebra 100, left and right transverse processes 105extending laterally from the posterior portion of vertebra 100, and leftand right pedicles 11. Each of the superior facets 43 has a superiorarticulating surface 145. The posterior portion of vertebra 100 also hasa pokerior arch (or lamina) 35, and a spinous process 46 that protrudesfrom the lamina 35 posteriorly, out of the page in FIG. 17 and to theleft in FIG. 18. In FIG. 17, the bony structure of the superior facets43 and the inferior facets 6 are intact, as it would be presented in avertebra without significant tissue degeneration or remodeling resultingfrom facet joint disease. Although the vertebra 100 is shown in FIG. 17as a generally structurally healthy and intact vertebra, if the vertebra100 were a diseased vertebra, the vertebra could exhibit signs of facetjoint disease. Consequently, structural pathology related to facet jointdisease would likely be visible. For example, the left superior facet 43and the right superior facet 43 of the vertebra 100 are symmetrical inFIG. 17 and FIG. 18. But in the case of a vertebra 100 with only onediseased joint, the facet on the diseased side would likely be showingpathological signs of disease such as tissue degeneration orinflammation resulting in an asymmetrical structural comparison betweenthe two facets.

Also, in more extreme cases the facet disease could progress to a statein which the articular process of the facet is eroded or inflamedresulting in anatomic morphology that is unique to the pathology of aparticular facet joint of an individual patient. This could presentunusual facet morphology that could be different from what is shown inFIGS. 17 and 18.

Furthermore, the facet disease could eventually disable the biomechanicsof a patient such that the facet joint is essentially non-articulatingand immobile. In this case, one superior facet of a first vertebra couldessentially be fused to one inferior facet of a second vertebra. Sincethe structural pathology of the diseased facet is variable, a surgeonmay determine that the best bone apposition surface or foundation forsecuring a facet implant is a resected bone surface.

Referring to FIG. 19 and FIG. 20 which are dorsal and lateral views ofthe same vertebra shown in FIG. 17 and FIG. 18 after a portion of theright superior facet 43 and a portion of the right inferior facet 6 havebeen resected. The removal of a portion of the superior facet 43 byresection results in a superior facet resection 111. In the resectionshown in FIG. 19 and FIG. 20, the superior resection 111 has tworesulting faces, a first resection surface 112 and a second resectionsurface 113. Likewise, the inferior facet resection results in aninferior facet resection surface 121.

Tissue removal tools (not shown) such as a bone burr, rasp, reamer,mill, saw, rounger, osteotome or similar tools designed to cut andremove bone tissue can be used to create these resection surfaces. Thesurgeon uses anatomic landmarks such as the pedicle 11 or transverseprocess 105 to align the tissue removal tools in such a way as to removethe portion of the facet necessary to provide a superior resection 111that serves as a bone apposition surface or foundation to eventuallysupport a superior facet prosthesis 300, as shown in FIG. 22. The leftsuperior facet 43 is shown intact in both FIG. 19 and FIG. 20, but aportion of the right superior facet 43 is resected resulting in thefirst resection surface 112 and the adjacent second resection surface113 (FIG. 19). The shape of the superior resection 111 will vary inaccordance with the structure of the tissue removal tool. In theembodiment shown in FIG. 19 and FIG. 20, the first resection surface 112and the second resection surface 113 are on approximately perpendicularplanes. However, the geometry of the resection surfaces is a function ofthe patient anatomy, the pathology of the diseased tissue, the techniqueof the surgeon, and other factors such as the type of tissue removaltools used to prepare the resection. In general, the first resectionsurface 112 will be formed in such a way that it will serve as afoundation to support the superior facet prosthesis 300 (FIG. 22). Thesecond resection surface 113 or other additional resection surfaces mayor may not be present.

FIG. 19 and FIG. 20 also show that a portion of the inferior facet 6 isresected by tissue removal instruments resulting in an inferiorresection surface 121. Such resection is preferably effected so thatresection is confined to the tissue of the inferior facet 6 and does notextend into the tissue of the posterior arch (or lamina) 35. In FIGS. 19and 20, the left inferior facet 6 is intact, while a portion of theright inferior facet 6 is resected resulting in the inferior resectionsurface 121 on the right side. The bone surrounding the inferiorresection surface 121 is contoured by tissue removal tools in a shapedesigned to cradle and support an inferior facet prosthesis 400 (FIG.23) on the medial side such that when the inferior facet prosthesis 400is loaded on the lateral side it compresses against and is supported bythe inferior resection surface 121.

Alternatively, the inferior facet 6 can be resected, and inferior facetprosthesis 400 sized and shaped, so that inferior facet prosthesis 400does not engage the inferior resection surface 121.

FIG. 21 is a dorsal view of the vertebra 100 with a fixation element 200placed through the superior resection 111 and into the bone of thepedicle 11 to receive the superior facet prosthesis 300 (FIG. 22). Thefixation element 200 is aligned and placed into the pedicle 11, similarto how other pedicle screws for posterior stabilization involved withvertebrae fusion are placed in the pedicle 11. In one method, a longguide wire (not shown), with a diameter sized to fit freely into acannulation 211 (as also shown in FIG. 26 and FIG. 27) in the fixationelement 200, is placed through the first resection surface 112 and intothe bone of the pedicle 11. The alignment of the long guide wire can beconfirmed by x-ray. The fixation element 200 is then guided over theguide wire and driven into the vertebra 100 by a driver (not shown)engaged with a drive feature 212 (FIG. 21) on a proximal post 230 of thefixation element 200. The fixation element 200 is driven into thevertebra 100 until a connection feature 213 (e.g., a screw thread) isjust above the first resection surface 112. This connection feature 213is eventually used to secure the superior facet prosthesis 300 to thevertebra 100.

In a second method for guiding the fixation element 200 into the pedicle11, a long guide wire (not shown), with a diameter sized to fit freelyinto a cannulation in a bone preparation instrument (not shown) such asa tap, drill, broach or reamer, is placed through the first resectionsurface 112 and into the bone of the pedicle 11. The alignment of thelong guide wire can be confirmed by x-ray. The bone preparationinstrument is then guided over the guide wire and driven into the boneof the pedicle 11 to prepare a cavity for the fixation element 200. Theguide wire and bone preparation instrument are then removed and thefixation element 200 is guided into the prepared cavity in the pedicle11 by a driver (not shown) engaged with the drive feature 212 on theproximal post 230 of the fixation element 200. Like in the first method,the fixation element 200 is driven into the vertebra until a connectionfeature 213 (e.g., a screw thread) is just above the first resectionsurface 112. This connection feature 213 is eventually used to securethe superior facet prosthesis 300 to the vertebra 100.

In yet a third method of placing the fixation element 200 in thepedicle, the surgeon aligns the fixation element 200 with anatomiclandmarks and simply drives the fixation element 200 through the firstresected surface 112 and into the pedicle 11. As with the first andsecond methods, the fixation element 200 is driven into the vertebra 100until a connection feature 213 (e.g., a screw thread) is just above thefirst superior resection surface 112.

In FIG. 22, a dorsal view illustrates a superior facet prosthesis 300placed around the fixation element 200. The superior facet prosthesis300 has a facet articulating component 320 that articulates against theinferior facet articulating surface of the vertebra above it. The facetarticulating component 320 is preferably formed in the general shape ofa blade or wing ear. The superior facet prosthesis 300 also has a boneapposition surface 322 that has been placed on the first resectionsurface 112 and an opening 324 in a flange 323 that surrounds thefixation element 200. The superior facet articulating component 320 hasan articulating surface 321 generally adjacent to the flange 323 that isoriented in a direction that faces approximately the same direction thatthe original anatomic superior articulating surface 145 faced prior toresection.

This orientation of the articulating surface 321 allows the superiorfacet prosthesis 300 to function as either a hemiarthroplasty implantand articulate against a natural anatomic inferior facet 6 or act as aportion of a unilateral prosthesis and articulate against an inferiorfacet prosthesis 400 on the vertebra superior (cephalad) to it. Noportion of superior facet prosthesis 300 rests on the lamina of thevertebra 100. In this application, a “unilateral prosthesis” is aprosthesis in which both facets of only one of the facet joints betweenadjacent vertebrae are replaced by prostheses. A “hemiarthroplasty” is atype of arthroplasty in which one side of an articulating joint surfaceis replaced with an artificial implant.

FIG. 23 is a dorsal view showing the addition of the inferior facetprosthesis 400 to the construct described in FIG. 22. The inferior facetprosthesis 400 generally has a shape similar to a longitudinal rod thatis curved to match the contour of the inferior resection 121 (FIGS. 19and 20). The inferior facet prosthesis 400 has an opening 410 throughits superior end 420 that is shaped to surround the portion of thefixation element 200 that protrudes from the first resection surface112. In FIG. 23, the inferior facet prosthesis 400 is placed over thesuperior facet prosthesis 300. However, the order of the placement ofthe prostheses 300, 400 can be reversed such that the inferiorprosthesis 400 is placed on the fixation element 200 first, followed bythe superior prosthesis 300. When only the inferior facet 6 or thesuperior facet 43 is being replaced, only the appropriate (superior orinferior) facet prosthesis 300 or 400 is placed on the fixation element200 without the other (inferior or superior) facet prosthesis 300 or400.

Because the various components of the implant are modular, manycombinations of configurations and implant size, structure and shapesare feasible. For example, in a patient with unusual anatomy, theinferior facet prosthesis 400 may need to be larger than expected toconform to a particularly unusual or exceptionally large morphology ofthe inferior resection surface 121, and the superior facet prosthesis300 may need to have an unusual angle to its articulating surface 321 toconform to particular anatomic constraints. If this is the case, themodularity of the system allows for the surgeon to assemble an implantspecifically designed to match the patient's anatomic structures duringthe surgery. This flexibility of a modular implant design allows theimplant manufacturer to accommodate a large variation in anatomicstructures with a limited selection of implant component sizes, shapes,and material types.

The modularity of the implant design also allows different components ofthe implant to be fabricated from different materials. Traditionally,bone fixation implants such as the fixation element 200 are fabricatedfrom biocompatible metals or alloys that provide sufficient strength andfatigue properties, such as cobalt chrome alloys, titanium and titaniumalloys, and stainless steels. However, the fixation element 200 may befabricated from ceramics, polymers, or biological materials such asallograft bone, composites, or other biocompatible structural materials.Likewise the superior facet prosthesis 300 and the inferior facetprosthesis 400 may be fabricated from metals, alloys, ceramics,polymers, biological materials, composites, or other biocompatiblestructural materials.

In FIG. 24, a dorsal view illustrates the addition of an enlarged head500 to the fixation element 200. The enlarged head 500 is tightened downto force the prostheses 300, 400 against the bone to stabilize them. Theenlarged head 500 shown in FIG. 24 has a hexagonal geometry on itsexternal surface that is shaped to accept a driver (not shown) that isused to force an internal connection feature 520 (e.g., a screw thread)of the enlarged head 500 onto the connection feature 213 of the fixationelement 200. In the case of the threaded embodiment of the connectionfeature 213, the enlarged head 500 is provided with a threadedconnection feature 520 and is driven onto the fixation element 200 byturning the enlarged head 500 and allowing the threads to drive allcomponents of the implant between the enlarged head 500 and the firstresection surface 112 against the bone at or near the resection surface112.

FIG. 25 is a perspective posterior view of the assembly of the fixationelement 200, the superior facet prosthesis 300, and the enlarged head500. The enlarged head 500 has been placed on the first resectionsurface 112.

FIG. 26 is a perspective, cross-sectioned view of the same constructshown in FIG. 25. The superior facet prosthesis 300, the enlarged head500, the fixation element 200, and the vertebra 100 have been cut by across-sectioning plane 150 placed along an axis that passes through thecenter of the fixation element 200. The cross-section plane 150 is shownfor visualization purposes to illustrate, using a cross-sectioned view,how the vertebra 100, fixation element 200, superior facet prosthesis300 and the enlarged head 500 engage each other. In actual surgery, itis highly unlikely that a surgeon would make a cut as illustrated by thecross-section 150 shown in FIG. 26.

FIG. 27 is a cranial, section view of the vertebra 100 and the implant,wherein the cross-section plane 150 is oriented to face the viewer. InFIG. 27, the fixation element 200 is in the vertebra 100. The embodimentof the fixation element 200 in FIG. 27 comprises a distal end 220 thatis shaped to guide the fixation element 200 into bone tissue, a bonestabilizing portion 210 adjacent to the distal end, a shaft portion 240adjacent to the bone stabilizing portion 210, a connection feature 213adjacent to the shaft portion 240, and a drive feature 212.

The distal end 220 shown in FIG. 27 has a frusto-conical shape thatallows the fixation element 200 to be driven or guided into the vertebra100. The distal end 220 could be shaped in the form of a spade tip,trochar tip, or twist drill tip to assist in the guidance of thefixation element 200 in the vertebra 100. The fixation element 200 mayalso have a cutting flute (not shown) formed in the distal end 220 tohelp remove bone tissue and accommodate the guidance of the fixationelement 200 in the vertebra 100. The bone stabilizing portion 210 helpsto secure the fixation element 200 to the vertebra 100. The bonestabilizing portion 210 can include various features designed to anchorinto bone such as threads, ribs, grooves, slots, fins, barbs, splines,bone ingrowth surfaces, roughened surfaces, or any geometric featurethat helps to engage the fixation element 200 with the bone tissue tohelp stabilize the fixation element 200. In FIG. 27, the bonestabilizing portion 210 has a unitary continuous bone thread 231.However, other types of threads such as multiple lead threads, variablepitched thread, non-uniform pitch thread, buttress thread, or otherthread forms used on bone screws may be used. Because FIG. 27 is across-sectional view, the full length of the cannulation 211 is seenpassing from the distal end 220 of the fixation element 200 to theproximal post 230 of the fixation element 200.

The drive feature 212 in the embodiment shown in FIG. 27 is an internalhex. However, any shape of drive feature 212 that transmits the loadsnecessary to drive the fixation element 200 into the vertebra 100 can beformed on the proximal post 230 of the fixation element 200. The depthof the drive feature 212 formed in the proximal post 230 of the fixationelement 200 is seen in the cross-sectional view of FIG. 27. The drivefeature 212 may be an internal drive feature such as the hex socketshown in this embodiment, or an external drive feature with geometry onthe periphery of the proximal post 230 of the fixation element 200 thatengages with a corresponding internal drive feature on a driver tool(not shown). In this embodiment the depth of the drive feature 212 isslightly longer than its cross-section is wide. This depth can beadjusted based on the material properties of the fixation element 200and the drive tool (not shown).

The fixation element 200 is fabricated from biocompatible base materialsthat provide the necessary structural rigidity and strength. Examples ofbase materials that may be used in the construction of the fixationelement 200 include titanium, titanium alloys, cobalt-chrome alloys,stainless steel alloys, zirconium alloys, other biocompatible metalmaterials, biocompatible ceramics, biocompatible composites, andbiocompatible polymers. The fixation element 200 may also have surfacematerials formed on the base material that provide material propertiesspecific to a particular portion of the fixation element 200. Forexample, the bone stabilization portion 210 could be coated withmaterials that allow for improved bone ingrowth into the implant surfacesuch as a hydroxylapatite, bioceramic, Bioglass®, or other calciumphosphate derived material. The tribological bearing properties of thematerial in the areas that the fixation element 200 interfaces withother artificial elements may be improved by applying surface hardeningtechniques to the material of the fixation element 200 in these areas.Surface hardening techniques known in the materials science andmaterials engineering arts such as anodizing, ion implantation, andother techniques could be applied to these isolated areas.

As mentioned previously, the connection feature 213 is formed on theportion of the fixation element 200 that protrudes from the firstresection surface 112. This connection feature 213 is designed toconnect the enlarged head 500 to the fixation element 200. In theembodiment of the connection feature 213 shown in FIG. 21, threads 260are on the external surface of this proximal section of the fixationelement 200. These threads 260 engage with the threads of the internalconnection feature 520 (FIG. 27) of the enlarged head 500. Although theconnection feature 213 in this embodiment is threaded, other mechanicallocking features (not shown) capable of locking the fixation element 200and the enlarged head 500 together, such as press fit, taper fit,bonding fit by cement or glue, interference fit, expansion fit andmechanical interlocking fit such as a bayonet connection, can be used asthe connection feature 213. A corresponding construction may then beused as connection feature 520 of the enlarged head 500.

Also shown in FIG. 27 is a cross-sectional view of the superior facetprosthesis 300. This embodiment of the superior facet prosthesis 300 hasa flange 323 that has an opening 324 that receives the fixation element200. In the assembled and implanted configuration of this embodiment,the flange 323 is positioned such that its bone apposition surface 322makes contact with the first resection surface 112. Although not shownin this embodiment, other embodiments of the superior facet prosthesis300 have structures (e.g., spikes) that protrude into the firstresection surface 112 to help resist torsion and other anatomic loads.Protruding from the flange 323 at a given angle α, and a given distanceX from the opening 324, is the articulating component 320. Thearticulating surface 321 of the facet articulating component 320replicates the natural articular surface of the replaced facet. Once thesurgeon assesses the anatomy of the superior facet 43 that is beingreplaced, a particular superior facet prosthesis 300 is selected thathas the angle α and the distance X that best fits the anatomy of thelevel of vertebra, the left or right side, and the size of the patient'sanatomy being replaced. Thus a kit containing various sizes and shapesof superior facet prostheses 300 is provided to the surgeon and thesurgeon selects the superior facet prosthesis 300 that best suits thesituation.

After the fixation element 200 and the superior facet prosthesis 300 areselected and placed, they are locked to the vertebra 100 by the enlargedhead 500. As shown in FIG. 24, the enlarged head 500 in this embodimenthas an internal connection feature 520 and a hexagonal shaped externaldrive feature 510 that is used to drive the enlarged head 500 over thefixation element 200 and against the superior facet prosthesis 300. Thespecific shape of the external drive feature 510 is dependent on themating shape of the driver (not shown).

Referring to FIG. 28, side and cross-sectional views illustrate sixdifferent embodiments of fixation elements, which are labeled A, B, C,D, E, and F. The figure shows a side view of each fixation elementembodiment and a cross-sectional view of each embodiment to the right ofthe respective side view. To the left of the six embodiments is arepresentative enlarged head 500. Embodiment A is the threaded fixationelement 200 embodiment shown in FIGS. 26 and 27 and described above.Embodiments B through E are various designs of fixation elements withnon-circular cross-sections. Embodiment B is a four rib cruciate designwith four longitudinal fins 285 configured to resist torsion when thefixation element 200 is in the vertebra 100. Embodiment C is an ovalshaped cross-section design that is wider in the first direction 286than the second direction 287 to resist torsion. If the width in thefirst direction 286 is equal to the width in the second direction 287,the cross-section shape becomes more of a circle and bone stabilizationportion 210 becomes more of a press-fit peg. Embodiment D is a squarecross-section design with four approximately perpendicular sides 288.The corners 289 of the sides 288 help to resist torsion. Embodiment E isa triangular cross-section design with three sides 291 to resisttorsion. Embodiment F is an anchor-like design that is driven into thevertebra, with the wire arches or barbs 290 being compressed against thehost bone and applying a radial expansion force so as to lock thestructure to the bone.

Referring to FIG. 28A, side and cross-sectional views illustrate sixmore different embodiments of fixation elements, which are labeled G, H,I, J, K, and L. FIG. 28A shows a side view of each fixation elementembodiment and a cross-sectional view of each embodiment to the right ofthe respective side view. Each embodiment has an attached or integrallyformed enlarged head 500′. Embodiment G is similar to the threadedfixation element 200 embodiment shown in FIGS. 10, 11, 12 and 24 anddescribed above. Embodiments H through K are various designs of fixationelements with non-circular cross-sections. Embodiment H is a four ribcruciate design with four longitudinal fins 285 configured to resisttorsion when the fixation element is in the vertebra 100. Embodiment Iis an oval shaped cross-section design that is wider in a firstdirection 286 than in a second direction 287 to resist torsion. If thewidth in the first direction 286 is equal to the width in the seconddirection 287, the cross-section shape becomes more of a circle and thebone stabilization portion 210 becomes more of a press-fit peg.Embodiment J is a square cross-section design with four approximatelyperpendicular sides 288. The corners 289 of the sides 288 help to resisttorsion. Embodiment K is a triangular cross-section design with threesides 291 to resist torsion.

Embodiment L is an anchor-like design that is similar to Embodiment F inFIG. 28, but with an attached or integrally formed enlarged head 500′.As embodiment L is driven into the vertebra, wire arches or barbs 290are compressed and apply radial expansion force against the wall of theprepared bone and into the pedicle 11, resulting in a locking anchor.

FIG. 29 is a perspective view of a radially expanding fixation element600. The radially expanding fixation element 600 comprises two mainelements, an expansion sleeve 620 and a central element 610 that isinside of the expansion sleeve 620. The radially expanding fixationelement 600 is placed into the vertebra 100 and then the central element610 is drawn outward relative to the expansion sleeve 620 resulting inradial expansion of the fixation element 600. This is shown in FIG. 30.

Referring to FIG. 30, side and bottom views illustrate the fixationelement 600 of FIG. 29. As a proximal post 630 of the central element610 is pulled axially along its longitudinal axis, and the expansionsleeve is held axially in the bone by compression fit, talons 621 on theexpansion sleeve 620 are radially expanded outward by a mandrel 660 onthe central element 610. The talons or fingers 621 provide bothtorsional and axial stability to the radially expanding fixation element600. This provides a secure fixation element for fixation of theremaining implant components. Furthermore, expansion of the fixationelement 600 may cause the fixation element 600 to center itself withinthe pedicle 11.

FIG. 31 is a perspective, cross-sectional view of a cross-pin element700 engaged with the fixation element 200 to help secure the fixationelement 200 both torsionally and axially. The cross-pin element 700 iscolumnar in shape having a distal end 710, a midsection 730 (with alength along its longitudinal axis that is longer than its transversecross-sectional width), and a proximal post 720. The distal end 710 isshaped to penetrate through bone tissue and into a cross hole 280 formedin the fixation element 200. Instrumentation (not shown) is used toalign the cross-pin element 700 with the cross-hole 280 via fixation ofthe instrumentation to the drive feature 212 or the cannulation 211 onthe fixation element 200 and alignment of the direction of insertion ofthe cross-pin element 700 with the cross-hole 280. Once the cross-pinelement 700 is in place in the bone and through the fixation element200, the torsional and axial stability of the fixation element 200 isimproved.

The various embodiments of the fixation element 200 described above andshown in FIG. 28 through FIG. 31 function in conjunction with theenlarged head 500 to hold the inferior facet prosthesis 400 and/or thesuperior facet prosthesis 300 to their respective resection surfaces112, 113, and/or 121. Various combinations of this modular implant willbe described below and shown in FIGS. 32 through 37. Although thesefigures illustrate the use of the fixation element 200 and the enlargedhead 500 as the mechanism for securing the prostheses 300, 400 to thevertebra 100, other clamping devices such as the screw fastener 17 (FIG.10) may be used to mount the prostheses 300, 400 to the bone. Forexample, the screw prostheses 17 shown in FIGS. 10 through 12 may passthrough either the opening 324 (FIG. 22) in the superior facetprosthesis 300 or the opening 410 (FIG. 23) in the inferior facetprosthesis 400 or through both of these openings 324, 410. The head ofthe screw fastener 17 acts as the securing mechanism by pressing theinferior facet prosthesis 400 and the superior facet prosthesis 300against their respective resection surfaces 112, 113, and/or 121.

FIGS. 32 through 37 demonstrate different combinations of assemblies offacet replacement prostheses. The basic components of the prosthesis arethe fixation element 200, the superior facet prosthesis 300, theinferior facet prosthesis 400, and the enlarged head 500. However, asdescribed above, a screw fastener 17 can replace the fixation element200 and the enlarged head 500.

Referring to FIG. 32, a dorsal view illustrates three sequential layersof vertebrae. A top vertebra 101 is above a middle vertebra 102, and themiddle vertebra 102 is above a bottom vertebra 103. Portions of some ofthe facets on the right side of the vertebrae are replaced byprostheses. With regard to the facet joint between the top vertebra 101and the middle vertebra 102, an inferior facet prosthesis 401 isarticulating against a superior facet prosthesis 302 to form anartificial unilateral joint. The inferior facet of the middle vertebra102 is replaced by an inferior facet prosthesis 402 and the superiorfacet of the bottom vertebra 103 is replaced by superior facetprosthesis 303. Thus, a second unilateral prosthetic joint is formedthat is also on the right side and is located at the level between themiddle vertebra 102 and the bottom vertebra 103. FIG. 32 demonstratesthe difference in shape of the inferior facet prosthesis 401 that isimplanted around the fixation element 201 without a superior facetprosthesis 300 and an inferior facet prosthesis 402 that is implantedaround a fixation element 202 and over a superior facet prosthesis 302.The opening 410 (not visible) of the inferior facet prosthesis 401 onthe top vertebra 101 in this assembly is offset more laterally than theopening 410 (not visible) in the inferior facet prosthesis 402 for themiddle vertebra 102. This is because the fixation element 201 isimplanted more laterally on the top vertebra 101 to preserve more of thesuperior facet since it is not replaced by a prosthesis at this level.

Referring to FIG. 33, a dorsal view illustrates the top vertebra 101 inintact form, without resection of the facets. Portions of both thesuperior and inferior facets on the right side of the middle vertebra102 are replaced by a superior facet prosthesis 302 and an inferiorfacet prosthesis 402. Only the right superior facet of the bottomvertebra 103 is replaced (i.e., by a superior facet prosthesis 303) inFIG. 33. Thus, a hemiarthroplasty replacement has been performed on theright facet joint between the top vertebra 101 and the middle vertebra102 and a unilateral replacement has been performed between the middlevertebra 102 and the bottom vertebra 103. The assembly shown in FIG. 33demonstrates how the superior facet prosthesis 302 can articulateagainst the natural inferior facet 6 and the superior facet prosthesis303 can articulate against the inferior facet prosthesis 402.

FIG. 34 is a dorsal view illustrating how the inferior facet prosthesis401 can articulate against the natural superior facet 43, or theinferior facet prosthesis 402 can articulate against the superior facetprosthesis 303. The right facet joint between the top vertebra 101 andthe middle vertebra 102 is a hemiarthroplasty replacement with theinferior facet replaced by the inferior facet prosthesis 401. The rightfacet joint between the middle vertebra 102 and the bottom vertebra 103is a unilateral replacement with the inferior facet replaced by theinferior facet prosthesis 402 and the superior facet of the bottomvertebra 103 replaced by the superior facet prosthesis 303.

Referring to FIG. 35, a dorsal view shows another example of how thesuperior facet prosthesis 303 can articulate against the naturalinferior facet 6 or the superior facet prosthesis 302 can articulateagainst the inferior facet prosthesis 401. In this assembly of theimplant, the right side between the top vertebra 101 and the middlevertebra 102 is a unilateral replacement and the right side between themiddle vertebra 102 and the bottom vertebra 103 is a hemiarthroplastyreplacement.

Referring to FIG. 36, a dorsal view shows another example of how theinferior facet prosthesis 402 can articulate against the naturalsuperior facet 43, or the inferior facet prosthesis 401 can articulateagainst the superior facet prosthesis 302. The right facet joint betweenthe top vertebra 101 and the middle vertebra 102 is a unilateralreplacement with the inferior facet of the top vertebra 101 replaced bythe inferior facet prosthesis 401 and the superior facet of the middlevertebra 102 replaced by the superior facet prosthesis 302. The rightfacet joint between the middle vertebra 102 and the bottom vertebra 103is a hemiarthroplasty replacement with the inferior facet replaced bythe inferior facet prosthesis 402.

Referring to FIG. 37, a dorsal view illustrates only one level, thatbetween the middle vertebra 102 and the bottom vertebra 103, beingreplaced on the right side. The right facet joint between the middlevertebra 102 and the bottom vertebra 103 is a unilateral replacementwith the inferior facet of the middle vertebra 102 replaced by theinferior facet prosthesis 402 and the superior facet of the bottomvertebra 103 replaced by the superior facet prosthesis 303.

FIG. 38 and FIG. 39 show two embodiments of the superior facetprosthesis. In FIG. 38, a perspective view illustrates an embodiment inwhich a curved superior facet prosthesis 305 with a curved articulatingcomponent 330 has a curved articulating surface 331. This curvedarticulating surface 331 allows for a more distributed contact loadbetween an inferior facet prosthesis, such as the inferior facetprosthesis 400 of FIG. 23, and the curved articulating surface 331. Thisallows slightly more flexibility in the position that the surgeon placesthe curved superior facet prosthesis 305 than the superior facetprosthesis 300 previously described. The articulating surface 321 of thesuperior facet prosthesis 300 previously described is relatively flat.The articulating surface 331 of the curved superior facet prosthesis 305is curved. Since the bearing portion of the inferior facet prosthesis400 is columnar, the two prosthesis can be aligned on a slight mismatchand make more of an anatomic contact if the articulated surface iscurved as in FIG. 38.

Referring to FIG. 39 a perspective view illustrates a bone ingrowthfeature 390 on a superior facet prosthesis 306. The bone ingrowthfeature 390 can be any surface that allows bone to grow into the implantbetween the first resection surface 112 of the vertebra 100 and theapposition surface 322 of the implant. Examples of bone ingrowthfeatures 390 include porous coating of beads or meshes,electrochemically etched shapes and porous pads pressed onto the implantsurface made from tantalum, titanium, cobalt chrome alloys and/or otherbiocompatible material such as hydroxylapatite or calcium phosphateceramics.

Referring to FIG. 40, a perspective view shows the inferior facetprosthesis 400, which is formed in the general shape of a finger ortalon. More particularly, the inferior facet prosthesis 400 is formedwith a flange 420 on its superior side shaped to fit between theenlarged head 500 and either the superior facet prosthesis 300 or thefirst resection surface 112. The flange 420 has an opening 410 that isdimensioned to allow the inferior facet prosthesis 400 to fit over theproximal post 230 of the fixation element 200 and around the shaftportion 240 of the fixation element 200. The inferior facet prosthesis400 also has an inferior portion 450 on the opposite side of the flange420 that has a bone apposition side 440 that is shaped to contact thesurface of the inferior facet resection surface 121 (FIG. 19) and ajoint articulation side 430 that is shaped to articulate against anatural or prosthetic superior facet.

Referring to FIG. 41, a perspective view shows an inferior facetprosthesis 460 also formed in the general shape of a finger or talon.The inferior facet prosthesis 460 is formed with a superior end 420having an opening 410 that is dimensioned and shaped to accept thefixation element 200. The inferior facet prosthesis 460 is generallycolumnar in shape, having a curved length designed to conform to theprepared anatomy of the vertebra 100. The inferior facet prosthesis 460of FIG. 41 has an inferior portion 470, which is shown opposite thesuperior end 420, and slightly medially offset from the superior end420. This medial offset of the opening 410 relative to the inferiorportion 470 allows the inferior facet prosthesis 400 to be anchored tothe bone by the fixation element 200 and secured to the bone by theenlarged head 500, or the superior facet prosthesis 300 in combinationwith the enlarged head 500, at an anatomical position that allowsoptimal bone fixation. The inferior facet prosthesis 460 of FIG. 41 hasa bone ingrowth surface 441 and a joint articulating side 430 on itsinferior end 470. In this embodiment, the bone ingrowth surface 441 is atextured structure that permits bone cells to grow into the implantsurface. The shape of the bone ingrowth surface 441 can be a uniformtextured surface as shown in FIG. 41, or can be a non-uniform randomizedstructure such as a open cell foam structure, a porous beaded structure,a wire mesh structure, an electrochemical etched structure, or otherbone ingrowth structures known in the design of orthopedic implants. Thebone ingrowth surface 441 is shaped to mate with the inferior resectedbone surface 121 shown in FIG. 19 and FIG. 20.

FIG. 42 shows an exploded, perspective view of the vertebra 100 with thesuperior facet prosthesis 300 installed. An additional locking washer800 is used to assist in stabilizing the attachment of the superiorfacet prosthesis 300 to the first resection surface 112. Theconstruction of the implant assembly shown in FIG. 42 is similar to thatof the assembly shown in FIG. 25 with the addition of the locking washer800 that is placed over and around the proximal post 230 of the fixationelement 200.

Referring to FIG. 43, a perspective view shows the same implant of FIG.42 with the enlarged head 500 locked onto the fixation element 200 andpushing the locking washer 800 against the superior facet prosthesis 300and into the bone tissue. This added bone penetration of the lockingwasher 800 helps to fix the superior prosthesis 300 such that the entireassembly is more mechanically stable with respect to the vertebra 100.

FIG. 43 shows a further step in the assembly of the implant constructdescribed in FIG. 42. In FIG. 43, the locking washer 800 is secured overthe fixation element 200 and into the bone tissue by the enlarged head500. Although this embodiment of the locking washer 800 is only shownwith the superior facet prosthesis 300, the locking washer 800 canalternatively be used to mechanically secure the inferior facetprosthesis 400, or the combination of the inferior facet prosthesis 400and the superior facet prosthesis 300. In the embodiment of the lockingwasher 800 shown in FIG. 42 and FIG. 43, the locking washer 800 isplaced over the superior facet prosthesis 300. However, the lockingwasher 800 may be placed under the superior facet prosthesis 300, underthe inferior facet prosthesis 400 and the superior facet prosthesis 300,or between the superior facet prosthesis 300 and the inferior facetprosthesis 400 to stabilize the implant construct.

FIG. 44 shows a perspective view of the locking washer 800. The lockingwasher 800 has a body 805 with an opening 810 that is dimensioned to fitover the proximal post 230 of the fixation element 200. The lockingwasher 800 also has an anti-rotation feature 820 that mates with eitherthe superior facet prosthesis 300 or the inferior facet prosthesis 400or a combination of both the inferior facet prosthesis 400 and thesuperior facet prosthesis 400. The anti-rotation feature 820 shown inthis embodiment is a flat surface, however, any feature that wouldrotationally constrain the locking washer 800 to the other components ofthe implant (such as a tab, groove, taper or other geometric shape) canbe formed on the locking washer 800 as an anti-rotation feature. Thelocking washer 800 also has prongs 830 that pass into the bone tissue ofthe vertebra 100 to help stabilize the implant construct. The prongs 830in this embodiment of the locking washer 800 are elongated protrusionsthat taper to a tissue penetration tip 840. The prongs have sidewalls850 that provide a surface to resist torsion once the locking washer 800penetrates the bone tissue. The prongs 830 may also be simple spikesthat are either symmetrical or asymmetrical in cross-section thatprotrude from the locking washer body 805. The shape and length of thelocking washer prongs 830 are dependent on how the locking washer 800 isused. The prongs 830 of the locking washer 800 that holds only one ofthe inferior facet prosthesis 400 or the superior facet prosthesis 300to the vertebra 100 may be shorter than prongs of a locking washer thatholds both the inferior facet prosthesis 400 and the superior facetprosthesis 300 to the vertebra 100.

FIG. 45 shows a perspective view of the superior facet prosthesis 300and inferior facet prosthesis 400 held to the vertebra 100 by anadjunctive flexible fixation element 900 and a secondary flexiblefixation element 910. These flexible fixation elements 900 and/or 910may be made from such constructs as suture, braided cable, wire, ribbon,and/or other constructs that have longer lengths than cross-sections andwithstand larger loads in tension than in compression. The flexiblefixation elements 900 and/or 910 may be manufactured from biocompatiblemetals, alloys such as cobalt chrome alloys, titanium alloys, stainlesssteel alloys, polymers, bioabsorbable materials, composites, or othermaterials that are biocompatible and can be formed into a flexibleelement structure 900 and/or 910 such as those shown in FIG. 45. Theadjunctive flexible element 900 shown in FIG. 45 is shown attached toand securing the elongated head 500. A flexible element attachmentportion 580 (e.g., including an opening) mates the flexible element 900to the elongated head. However, the adjunctive flexible fixation element900 may alternatively or additionally be attached to the fixationelement 200, the superior facet prosthesis 300, the inferior facetprosthesis 400 or any combination of the above listed elements. Aflexible fixation attachment portion 480 (e.g., including an opening) inthe inferior facet prosthesis 400 allows the secondary flexible fixationelement 910 to secure the inferior facet prostheses 400 to the vertebra100. The flexible fixation elements 900 and/or 910 may be secured to thevertebra 100 by physically wrapping them around anatomic features suchas the posterior arch 35, the spinous process 46, transverse process105, or a combination of these anatomic features. The flexible element900 and the secondary flexible element 910 may also be secured to thevertebra 100 by bone anchors such as anchors designed to anchor flexiblefixation elements (such as suture, not shown) to bone. Suture anchorssuch as threaded suture anchors, barbed suture anchors, toggle sutureanchors or any other means of anchoring a flexible fixation element tobone may be used to anchor the flexible fixation element 900 and/or thesecondary flexible fixation element 910 to the vertebra 100.

FIG. 46 is a dorsal view of a bilateral inferior facet prosthesis 1000.The bilateral inferior facet prosthesis 1000 is a one-piece inferiorfacet prosthesis that has both a right inferior side 1040 and a leftinferior side 1020 connected by a stabilizing bar 1010. Both the rightinferior side 1040 and the left inferior side 1020 are designed to fixto the top vertebra 101 at the respective inferior resection surface 121(FIG. 19) and at the first resection surface 112. The bilateral inferiorfacet prosthesis 1000 allows replacement of both the left and the rightinferior facets. In this embodiment, the bilateral inferior facetprosthesis 1000 is placed over the left and right fixation elements 200which extend into the bone of the top vertebra 101. In the embodimentshown in FIG. 46, the right inferior side 1040 is articulating againstthe right superior facet prosthesis 300 attached to the bottom vertebra102. Also in this embodiment, the left inferior side 1020 isarticulating against the left natural superior facet 43 of the bottomvertebra 102. The stabilizing bar 1010 of the bilateral inferiorprosthesis 1000 is designed to stabilize the left side 1020 and theright side 1040 so that they are secure.

FIG. 47 illustrates a perspective view of a superior facet prosthesis1100 coupled to the vertebra 3. The superior facet prosthesis 1100 has abone apposition surface (not shown) that has been placed on a firstresection surface 1112 and an opening (not shown) in a flange 1116 thatsurrounds a fixation element 1110, and coupled thereto by a lockingfastener such as a castle nut 1114 or the like. The superior facetprosthesis 1100 has a superior facet articulating component 1120 with anarticulating surface 1122 generally adjacent to the flange 1116. Thearticulating surface 1122 is oriented in a direction that facesapproximately the same direction that the original anatomic superiorarticulating surface faced prior to resection. This orientation of thearticulating surface 1122 allows the superior facet prosthesis 1100 tofunction as either a hemiarthroplasty implant by articulating against anatural anatomic inferior facet 6 or as a unilateral prosthesis byarticulating against an inferior facet prosthesis on the vertebrasuperior (cephalad) to it, such as the inferior facet prosthesis 4 shownin FIG. 5, the inferior facet prostheses 10 shown in FIGS. 8 and 9, andthe inferior facet prosthesis 400 shown in FIG. 40, as well as thosedescribed below.

The facet articulating component 1120 is preferably formed in thegeneral shape of a blade or wing ear, wherein the articulating surface1122 has a concave shape. In the embodiment shown, the articulatingsurface 1122 curves from an orientation generally perpendicular to theflange 1116 towards an orientation generally parallel to the flange 1116from a distal end 1124 thereof to a proximal end 1126 thereof.

The concave shape of the articulating surface 1122 provides moretolerance for a miss-match with the natural anatomic inferior facet 6 orwith the inferior facet prosthesis 4 on the vertebra superior to it.Functionally, the clearance between the concave shape of thearticulating surface 1122 and the adjacent inferior facet 6 or inferiorfacet prosthesis 4 increases as the patient bends forward (flexion) anddecreases as the patient bends backward (extension). Thus in flexion thepatient has more facet movement allowing for more torsion (twisting) andlateral bending (side to side movement) than in a neutral stance. As thepatient extends, the articulating members are more constrained intorsion and lateral bending. This mimics the natural anatomicconstraints of the spinal facets.

FIG. 48 is a perspective view of the same construct shown in FIG. 47,but with the implants and the vertebra 3 cut by a cross-sectioning plane1130 placed along an axis that passes through the center of the fixationelement 1110. The cross-section plane 1130 shown cutting through thevertebra 3 and the implant of FIG. 47 is shown for visualizationpurposes to illustrate, using a cross-sectioned view, how the vertebra3, fixation element 1110, and superior facet prosthesis 1100 engage eachother.

The fixation element 1110 provides a mechanism that affixes the superiorfacet prosthesis 1100 to vertebra 3. Fixation element 1110 is implantedinto the interior bone space of the left pedicle 11 (FIG. 6) on thevertebra 3 and may or may not extend into the vertebral body of vertebra3 to provide additional stability. The fixation element 1110 can takethe form of a screw (as shown), or any of the devices shown in FIGS.28-30. The fixation element 1110 has a drive feature 1140, which is aninternal hex in the embodiment shown in FIG. 48. However, any shape ofdrive feature that transmits the loads necessary to drive the fixationelement 1110 into the vertebra 3 can be formed on a proximal post 1142of the fixation element 1110.

The depth of the drive feature 1140 formed in the proximal post 1142 ofthe fixation element 1110 is seen in the cross-sectional view of FIG.48. The drive feature 1140 may be an internal drive feature such as thehex socket shown in this embodiment, or an external drive feature withgeometry on the periphery of the proximal post 1142 of the fixationelement 1110 that engages with a corresponding internal drive feature ona driver tool (not shown). The flange 1116 of the superior facetprosthesis 1100 is secured to the fixation element 1110 by the castlenut 1114 or the like.

The flange 1116 of the superior facet prosthesis 1100 includes acoupling portion 1144 having a generally semispherical bone engagingsurface 1150 on the apposition side of the superior facet prosthesis1100 that engages a corresponding semispherical resection 1146 in thebone bed of the pedicle of vertebra 3. The term “semispherical” relatesto a surface that includes some sectorial portion of a sphere, which maybe less than a hemisphere. A semispherical surface may be concave orconvex. A surface that is semispherical or generally semispherical mayhave some deviations from a precise semispherical shape.

The semispherical resection 1146 may be said to be “inversely shaped”with respect to the coupling portion because the semispherical resection1146 has a generally concave surface that matches the generally convexsurface of the coupling portion 1144. Although the coupling portion 1144and the semispherical resection 1146 are semispherical in the embodimentof FIGS. 47 and 48, in alternative embodiments, they may have a varietyof other matched shapes, including three-dimensional parabolas,ellipsoids, and other regularly or irregularly curved or flat-sidedshapes. Furthermore, although the coupling portion 1144 is convex andthe semispherical resection 1146 is concave in the embodiment of FIGS.47 and 48, in alternative embodiments, the shapes may be reversed sothat a coupling portion is concave and a resection is convex.

In the embodiment of FIGS. 47 and 48, the coupling portion 1144 isintegrally formed with the articulating surface 1122 of the superiorfacet articulating component 1120. The coupling portion 1144 may be saidto be “attached to” the articulating surface 1122 because in thisapplication, the term “attached” is used broadly to include parts thatare integrally formed with each other as well as parts that are formedseparately and subsequently coupled together.

The semispherical resection 1146 in the bone bed allows for lesstransverse process to be resected (vs. a flat bone bed resection). Thesemispherical resection 1146 in the bone bed also allows for more stablesupport of the superior facet prosthesis 1100, than does a flat bone bedresection, as the superior facet prosthesis 1100 is polyaxiallysupported in such a way as to resist any shear forces applied betweenthe semispherical resection 1146 and the coupling portion 1144. In thisapplication, “polyaxial” refers to a linear or angular force or motionacting with respect to at least two perpendicular axes. The couplingportion 1144 may seat directly against the semispherical resection 1146.In this application, an item that “seats directly against” another ispositioned to abut the other item so that surfaces of the two items arein contact with each other.

The coupling portion 1144 has a fixation element receiving aperture 1148that can be made slightly larger than a circumferential diameter of thefixation element 1110 taken in a direction perpendicular to alongitudinal axis thereof to provide accurate polyaxial seating of theimplant 1100 in relation to the resected bone bed and fixation element1110, as well as to provide increased tolerance for miss-match. Animplant engaging end 1154 of the castle nut 1114 (or other fastener)also has a semispherical shape for engaging a semispherical nut engagingside of the coupling portion 1144 of the superior facet prosthesis 1100at the final position of the superior facet prosthesis 1100.

The semispherical shape of the coupling portion 1144 enables thecoupling portion 1144 to move polyaxially against the semisphericalresection 1146. Movement “against” the semispherical resection refers tomovement in which the coupling portion 1144 remains substantiallycontinuously in contact with the semispherical resection 1146 so as toslide against the semispherical resection 1146. Accordingly, duringinstallation, a surgeon can position the coupling portion 1144 againstthe semispherical resection 1146 and then pivot the coupling portion1144 along three perpendicular axes, without removing the couplingportion 1144 from the semispherical resection 1146. The coupling portion1144 simply rotates against the semispherical resection 1146.

The phrase “polyaxial motion” refers to any combination of translationand/or rotation along at least two perpendicular axes. Since thecoupling portion 1144 is pivotable with respect to the semisphericalresection 1146 along three perpendicular axes, the coupling portion 1144is “tri-axially pivotable” with respect to the semispherical resection1146.

When the superior facet prosthesis 1100 has been rotated to the properorientation, the articulating surface 1122 is positioned for properarticulation against the corresponding inferior facet or inferior facetprosthesis. The orientation of the coupling portion 1144 may then befixed with respect to the semispherical resection 1146 by tightening thecastle nut 1114 (or another fastener) on the fixation element 1110,thereby firmly gripping the coupling portion 1144 against thesemispherical resection 1146. Accordingly, the coupling portion 1144 is“selectively polyaxially movable” with respect to the semisphericalresection 1146 because the coupling portion 1144 is movable with respectto the semispherical resection 1146 along multiple perpendicular axesuntil the surgeon decides to fix its disposition.

In alternative embodiments (not shown) of the invention, tri-axialpivotal movement need not be provided. Rather, a coupling portion and acorresponding resection surface may have a cylindrical, flat-sided,splined, or other shape designed to enable relative translation inaddition to or in place of rotation. In place of the fixation elementreceiving aperture 1148, an elongated fixation element receivingaperture may be used to accommodate relative translation between thecoupling portion and a fixation element. Alternatively, a couplingportion and a resection surface may be shaped to provide relativepivotal motion along only one or two axes.

In an alternative embodiment the implant engaging end 1154 of the castlenut 1114 (or other fastener) can be deformable such that the implantengaging end 1154 conforms under pressure to the adjacent surface of thecoupling portion 1144 regardless of the angle of the surface withrespect to the axis of the castle nut 1114. The deformable end can beformed of a plastic such as polyethylene attached to the metal body ofthe castle nut 1114, but is preferably formed of a substance thathardens over time, such as a fast-curing and biocompatible resin or amaterial that is heated prior to insertion into the patient and hardensupon cooling to the patient's body temperature. The material thathardens over time provides more stability than the deformable material,though both provide acceptable results.

FIG. 48 also shows an angled resection 1112 and corresponding angledflat 1156 on the apposition side of the superior facet prosthesis 1100in combination with the semispherical resection 1148.

The surfaces of the apposition side of the coupling portion 1144 andflat 1156, as well as fixation element 1110, may or may not includeporous coatings to facilitate bone ingrowth to enhance the long-termfixation of the implant. Furthermore, such porous coatings may or maynot include osteoinductive or osteoconductive substances to furtherenhance the bone remodeling into the porous coating.

FIG. 49 shows a perspective view of the vertebra 3 with a fixationelement 1110 portion implant placed through the semispherical resection1146 in the resection surface 1112 and into the bone of the pedicle 11.The fixation element 1110 is aligned and placed into the pedicle 11 in amanner similar to that of other pedicle screws for posteriorstabilization vertebrae fusion procedures.

In FIG. 50, a perspective view illustrates the superior facet prosthesis1100 in place around the fixation element 1110. The castle nut 1114 hasnot yet been installed. As shown, the coupling portion 1144 has asemispherical nut engaging surface 1152.

FIG. 51 is a top view of the superior facet prosthesis 1100,particularly showing the curved shape of the articulating surface 1122and the semispherical bone engaging surface 1150 of the coupling portion1144. Additionally, FIG. 51 more clearly illustrates the angled flat1156 on the apposition side of the superior facet prosthesis 1100.

FIG. 52 is an illustration of a rear view of the superior facetprosthesis 1100. In this context, “rear” means as viewed from along theaxis of the fixation element receiving aperture 1148. FIG. 52particularly shows the curved shape of the articulating surface 1122 andthe semispherical nut engaging surface 1152 of the coupling portion1144.

FIG. 53A shows a kit including a plurality of differently configuredsuperior facet prostheses 1100, 1162, 1164, 1166, 1168. View A is a rearview of the superior facet prostheses 1100, 1162, 1164, 1166, 1168,while View B illustrates a perspective view of the laterally adjacentprosthesis 1100, 1162, 1164, 1166, 1168 rotated 90°. As shown, thesuperior facet prostheses 1100, 1162, 1164, 1166, 1168 have differingphysical dimensions.

Referring again to FIG. 51, which shows a single superior facetprosthesis 1100, some of the physical dimensions that change between thedifferently sizes superior facet prostheses 1100, 1162, 1164, 1166, 1168in the kit (FIG. 53A) are a resection angle (α), an x offset (X₁), a yoffset (Y₁), a facet angle (β), and a facet articulation radius (R).Exemplary values for the foregoing will be provided below. Although theexemplary values relate primarily to L5 superior and L4 inferior, theymay apply to other combinations of vertebrae in the lower back and/orthe sacrum. One or more of these variables can change between thedifferent superior facet prosthesis sizes.

P1 is the most medial and anterior point on the articulating surface1122. The superior pedicle axis 1170 is the axis that is colinear withthe longitudinal axis of the fixation element 1110 that is positionedthrough the pedicle 11 nearest to the resected superior facet (notshown). The superior pedicle axis 1170 extends through a saddle pointS1, which is offset as shown, by an offset 1176, which may be about 2mm, from the fixation element receiving aperture 1148. The superiorpedicle axis 1170 is parallel with the direction of the y offset (Y₁).The direction of the x offset (X₁) is perpendicular to the direction ofthe y offset (Y₁). The direction of the x offset (X₁) is generally, butnot precisely, lateral to medial with respect to the central axis of thepatient's spine.

P4 is the most posterior point on the articulating surface 1122. Asshown, P4 is displaced from the saddle point S1 by an x offset (X₄) anda y offset (Y₄). The direction of the X₄ offset is parallel to that ofthe X₁ offset, and the direction of the Y₄ offset is parallel to that ofthe Y₁ offset.

The resection angle (α) for the superior facet prostheses 1100 can rangefrom 5° to 85°. However, the optimal range of the resection angle (α)for the majority of patients will range from 30° to 70°. Thus, by way ofexample, a family containing nine sets of superior facet prostheses 1100can be provided with the resection angles (α) varying in increments of5°. Sets of superior facet prostheses 1100 would be provided withresection angles (α) at 30°, 35°, 40°, 45°, 50°, 55°, 60°, 65° and 70°.

The x offset (X₁) for the superior facet prosthesis 1100 can range from5 mm to 30 mm. However, for the majority of patients, the x offset (X₁)will range from 10 mm to 20 mm. Therefore a family of superior facetprostheses 1100 can be provided with the x offset (X₁) varying inincrements of 5 mm. Thus, sets of superior facet prostheses 1100 wouldbe provided with x offset (X₁) at 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, and20 mm to provide superior facet prostheses 1100 that cover thestatistical range for the majority of the population of patients needingsuperior facet prostheses 1100.

The y offset (Y₁) for the superior facet prosthesis 1100 can range from2 mm to 20 mm. However, for the majority of patients, the y offset (Y₁)will range from 5 mm to 15 mm. Therefore a family of superior facetprostheses 1100 can be provided with the y offset (Y₁) varying inincrements of 2 mm. Thus, sets of superior facet prostheses 1100 wouldbe provided with y offset (Y₁) at 5 mm, 7 mm, 9 mm, 11 mm, 13 mm, and 15mm to provide superior facet prostheses 1100 that cover the statisticalrange for the majority of the population of patients needing superiorfacet prostheses 1100.

The x offset (X₄) for the superior facet prosthesis 1100 can range fromabout 5 mm to about 25 mm. However, for the majority of patients, X₄will range from about 8 mm to about 20 mm. A family of superior facetprostheses may be provided with X₄ values varying in increments of 2 mm.Thus, sets of superior facet prostheses 1100 would be provided with X₄values of 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, and 20 mm to providesuperior facet prostheses 1100 that cover the statistical range for themajority of the population of patients needing superior facet prostheses1100.

The y offset (Y₄) for the superior facet prosthesis 1100 can range fromabout −5 to about 15 mm. However, for the majority of patients, Y₄ willrange from about −2 mm to about 10 mm. A family of superior facetprostheses may be provided with Y₄ values varying in increments of 2 mm.Thus, sets of superior facet prostheses 1100 would be provided with Y₄values of −2 mm, 0 mm, 2 mm, 4 mm, 6 mm, 8 mm, and 10 mm to providesuperior facet prostheses 1100 that cover the statistical range for themajority of the population of patients needing superior facet prostheses1100.

The facet angle (β) for the superior facet prosthesis 1100 can rangefrom 50° to 120°. However, for the majority of patients, the facet angle(β) will range from 60° to 100°. Therefore a family of superior facetprostheses 1100 can be provided with the facet angle (β) varying inincrements of 5°. Thus, sets of superior facet prostheses 1100 would beprovided with the angle (β) at 60°, 65°, 70°, 75°, 80°, 85°, 90°, 95°,and 100° to provide superior facet prostheses 1100 that cover thestatistical range for the majority of the population of patients needingsuperior facet prostheses 1100.

Once the surgeon assesses the anatomy of the superior facet that isbeing replaced, a particular superior facet prosthesis 1100 is selectedthat has the curvature and overall angle of the articulating surface1122, with respect to the flange 1116 that best fits the anatomy of thelevel of vertebra, the left or right side, and the size of the patient'sanatomy being replaced. Thus a kit containing various sizes and shapesof superior facet prostheses 1100 is provided to the surgeon and thesurgeon selects the superior facet prosthesis 1100 that best suits thesituation.

According to one example, such a kit may contain nine prostheses, whichmay be dimensioned to provide a variety of combinations of values for α,X₁, Y₁, β, X₄, Y₄, and R, within the ranges listed above. If desired,one or more of the above-listed variables may remain constant over theentire kit. For example, R may have a constant value, such as 11.5 mm,for all members of the kit.

The prostheses 1100, 1162, 1164, 1166, 1168 of the kit of FIG. 53A arenot simply scaled up or down, but are varied according to a number ofcarefully selected parameters to cover the vast majority of morphologiesoccurring in the L5 vertebra. In a similar manner, a plurality ofinferior facet prostheses adapted to replace inferior facets can beprovided either as a separate kit, or in combination with the kit ofFIG. 53A. Such a kit will be shown and described in connection with FIG.53D.

FIGS. 53B and 53C illustrate top and side views, respectively, of anexemplary inferior facet prosthesis 1172. The inferior facet prosthesis1172 has an x offset (X), a y offset (Y), and a z offset (Z), which areillustrated in FIGS. 53B and 53C. As shown, the offsets X, Y, and Z runbetween a saddle point S1 of the inferior facet prosthesis 1172 and acenter point C1 of the articulation surface 1174. The saddle point S1 ofFIGS. 53B and 53C is defined in a manner similar to that of the superiorfacet prosthesis 1100 of FIG. 51.

As shown in FIGS. 53B and 53C, the inferior facet prosthesis 1172 has asemispherical coupling portion similar to the coupling portion 1144 ofthe superior facet prosthesis 1100 introduced in the description of FIG.47. Accordingly, the inferior facet prosthesis 1172 provides the sametype of tri-axial pivotal motion during installation as the couplingportion 1144, as described previously. The coupling portion of theinferior facet prosthesis 1172 may also be nested in the couplingportion 1144 of the superior facet prosthesis 1100, or vice versa, toenable independent polyaxial adjustment of the prostheses 1100, 1170when positioned in engagement with a single semispherical resection1146.

Referring to FIG. 53D, a perspective view illustrates a kit of inferiorfacet prostheses 1180, 1182, 1184, 1186, 1188, 1190. Again, the physicaldimensions can vary between the various inferior facet prostheses 1180in the kit of FIG. 53D. These dimensions may include an inferiorresection angle (Iα), an inferior x offset (X), an inferior y offset(Y), an inferior facet angle (Iβ), an inferior facet articulation radius(IR), and an inferior z offset (Z, from the center of fixation to thecenter of the articulation radius).

The inferior resection angle Iα is the angle of the flat resection to bemade in the vertebra, for example, the vertebra 101 illustrated in someof the preceding drawings, to serve as a backing for the articulatingsurface of the selected inferior facet prosthesis 1180, 1182, 1184,1186, 1188, or 1190. When measured according to the coordinate systemestablished for the superior facet prosthesis 1100, as illustrated inFIG. 51, the inferior resection angle Iα may be approximately the sameas the facet angle β for the superior prosthesis 1100 because thearticulation surfaces 1122, 1174 are to be positioned generally parallelto each other. Due to the clearance between the articulating surfaces1122, 1174 and the generally concave and convex shapes thereof, as longas the selected inferior facet prosthesis 1180, 1182, 1184, 1186, 1188,or 1190 is placed so that the articulating surfaces 1122, 1174 aregenerally parallel to each other, proper articulation may be expected tooccur.

Thus, the inferior facet prosthesis 1180, 1182, 1184, 1186, 1188, or1190 may be dimensioned such that Iα is nearly the same as β, and theorientation of the articulating surface 1174 may be adjusted as neededto permit the inferior facet prosthesis 1180, 1182, 1184, 1186, 1188, or1190 to be attached to the corresponding vertebra 101. Accordingly, Iαneed not be determined based on measurement of the vertebra 101, but mayinstead be inferred based on the selection of the superior facetprosthesis 1100, 1162, 1164, 1166, or 1168 and adjusted duringinstallation.

The inferior facet angle Iβ may be defined as the angle of the surfaceto which the articulating surface 1174 is most nearly parallel. Due tothe shape of the inferior facet prostheses 1180, 1182, 1184, 1186, 1188,or 1190, this angle is the same as the inferior resection angle Iα, whenmeasured according to the coordinate system of the superior facetprosthesis 1100 of FIG. 51.

The inferior pedicle axis 1170 is the axis that is collinear with thelongitudinal axis of the fixation element 1110 that is positionedthrough the pedicle 11 nearest to the resected inferior facet (notshown). This axis is parallel with the direction of the inferior yoffset (Y). The direction of the inferior x offset (X) is perpendicularto the direction of the inferior y offset (Y). The direction of theinferior x offset (X) is generally lateral to medial with respect to thecentral axis of the patient's spine. The direction of the inferior yoffset (Y) is generally anterior to posterior. The direction of theinferior z offset (Z) is generally cephalad to caudal.

The inferior x offset (X) for the inferior facet prosthesis 1180 canrange from 0 mm to 20 mm. However, for the majority of patients, theinferior x offset (X) will range from 2 mm to 16 mm. Therefore a familyof inferior facet prostheses 1180 can be provided with the inferior xoffset (X) varying in increments of 2 mm. Thus, sets of inferior facetprostheses 1180 would be provided with inferior x offset (X) at 2 mm, 4mm, 6 mm, 8 mm, 10 mm, 12 mm, 14 mm, and 16 mm to provide inferior facetprostheses 1180 that cover the statistical range for the majority of thepopulation of patients needing inferior facet prostheses 1180.

The inferior y offset (Y) for the inferior facet prosthesis 1180 canrange from −15 mm to 5 mm. However, for the majority of patients, theinferior y offset (Y) will range from −12 mm to 4 mm. Therefore a familyof inferior facet prostheses 1180 can be provided with the inferior yoffset (Y) varying in increments of 2 mm. Thus, sets of inferior facetprostheses 1180 would be provided with inferior y offset (Y) at −12 mm,−10 mm, −8 mm, −6 mm, −4 mm, −2 mm, 0 mm, 2 mm, and 4 mm to provideinferior facet prostheses 1180 that cover the statistical range for themajority of the population of patients needing inferior facet prostheses1180.

The inferior facet articulation radius (IR) for the inferior facetprosthesis 1180 can range from 5 mm to 30 mm. However, for the majorityof patients, the inferior facet articulation radius (IR) will range from10 mm to 15 mm. A family of incremented inferior prostheses may beprovided to cover the aforementioned range. Alternatively, the inferiorfacet articulation radius (IR) may be set at a given value, for example,12 mm, and such a value may be used in substantially all cases.

The inferior z offset (Z) for the inferior facet prosthesis 1180 canrange from 20 mm to 40 mm. However, for the majority of patients, theinferior z offset (Z) will range from 25 mm to 31 mm. Therefore a familyof inferior facet prostheses 1180 can be provided with the inferior zoffset (Z) varying in increments of 1 mm. Thus, sets of inferior facetprostheses 1180 would be provided with inferior z offset (Z) at 25 mm,26 mm, 27 mm, 28 mm, 29 mm, 30 mm, and 31 mm to provide inferior facetprostheses 1180 that cover the statistical range for the majority of thepopulation of patients needing inferior facet prostheses 1180.

If desired, a kit having ten inferior facet prostheses may be assembled.Like the prostheses 1100, 1162, 1164, 1166, 1168 of the kit of FIG. 53A,the prostheses 1180, 1182, 1184, 1186, 1188, 1190 of FIG. 53D are notsimply scaled up or down, but are varied according to a number ofcarefully selected parameters to cover the vast majority of morphologiesoccurring in the L4 vertebra and/or other vertebrae.

The parameters of the prostheses 1100, 1162, 1164, 1166, 1168 of FIG.53A and/or the prostheses 1180, 1182, 1184, 1186, 1188, 1190 of FIG. 53Dmay include at least two dimensions that vary among the members of thekit independently of each other. Dimensions that vary independently ofeach other need not change according to any established relationshipbetween the dimensions, but instead, one may change while the otherremains the same between any two prostheses of the kit.

FIG. 53E is a perspective view illustrating how a superior facetprosthesis 1100 and an inferior facet prosthesis 1180 fit together. Thesurgeon selects an inferior facet prosthesis that, in addition to mostadequately meeting the anatomy of the patient, has an articulatingsurface adapted for articulating with the articulating surface of thesuperior facet prosthesis selected.

FIG. 53F is a dorsal view of a superior facet prosthesis 1100 and aninferior facet prosthesis 1204 attached to the L5 and L4 lumbarvertebrae 102, 101. In FIG. 53F, the superior facet prosthesis 1100 isattached to the left side of the L5 vertebra 102 and the inferior facetprosthesis 1204 is attached to the left L4 vertebra 101. The twoprostheses 1100, 1204 are positioned on respective bone resections andoriented such that they articulate together through the range of motionnaturally exhibited between the L4 and L5 vertebrae 1100, 1204. Thisrange of motion includes flexion-extension, lateral left and rightbending, torsion along a sagittal axis and combinations and coupling ofall these ranges of motion.

FIG. 53F shows the prostheses 1100, 1204 and vertebrae 101, 102 in anatural position. The articulation surface 1174 of the inferiorprosthesis 1204 and the articulation surface 1122 of the superiorprosthesis 1100 are in contact in the neutral position. However, theprostheses 1100, 1204 are shaped to allow anatomic contact andarticulation between the inferior facet articulation surface 1174 andthe superior facet articulation surface 1122 throughout various anatomicranges of motion.

Also shown in FIG. 53F are two planes labeled “Plane 1” and “Plane 2”that that intersect along an axis (not shown) that passes through thecontact areas of the superior facet articulation surface 1122 and theinferior facet articulation surface 1174. Plane 1 is parallel to thepage of FIG. 53F, and Plane 2 is perpendicular to the page.

FIG. 53G is a posteriolaterial view of the same inferior and superiorfacet prostheses 1100, 1204 with the planes shown in FIG. 53F. In FIG.53G, Plane 2 is oriented parallel to the page and plane 1 is orientedperpendicular to the page. FIG. 53G illustrates the saddle point (S1) ofthe vertebra 102 to which the superior facet prosthesis 1100 is coupled,and the saddle point (S2) of the vertebra 101 to which the inferiorfacet prosthesis 1204 is coupled. The saddle points S1, S2 are displacedfrom each other along an x offset (IX) parallel to the axis at whichPlane 1 and Plane 2 intersect, a y offset (IY) extending perpendicularto Plane 2, or out of the page with respect to FIG. 53G, and a z offset(IZ) extending perpendicular to Plane 1. The offsets IX, IY, and IZ maybe used for implant sizing and/or selection, as will be discussedsubsequently.

FIG. 53H is a posteriolateral view showing a cross-section along Plane2. This cross-section view cuts through the articulation surfaces 1122,1174 of the prostheses 1100, 1204, thereby showing the convex shape ofthe inferior articulation surface 1174 against the concave shape of thesuperior articulation surface 1122.

FIG. 53H also illustrates the cephalad and caudal ends 1250, 1252 of thearticulation surface 1122 of the superior facet prosthesis 1100. Thearticulation surface 1122 has a radius of curvature 1254 generally aboutan axis 1256. However, since the radius of curvature 1254 changes alongthe articulating surface 1122, the axis 1256 may be the center ofcurvature for only a portion of the articulation surface 1122. Theradius of curvature 1254 is shown extending from the axis 1256 to thearticulation surface 1122 in FIG. 53H. Furthermore, FIG. 53H illustratesa longitudinal axis 1258 of the spine in general. The axis 1256 isangled from the axis 1258 by an offset angle 1259. Since the axis 1256and the axis 1258 may not both be precisely parallel to Plane 2, theoffset angle 1259 may have a component that extends out of the page withrespect to the view of FIG. 53H.

FIG. 53I is a cephalad view showing a cross-section along Plane 1. Thiscross-section cuts through the articulation surfaces 1122, 1174 of theprostheses showing the convex shape of the inferior articulation surface1174 against the concave shape of the superior articulation surface1122. Each of the articulating surfaces 1122, 1174 has a curved shape.The articulating surfaces 1122, 1174 of the superior and inferiorprostheses 1100, 1204, respectively, are shaped and relativelypositioned to articulate against each other such that a medial-lateralrange of relative motion between the first and second vertebrae 101, 102increases significantly with flexion (i.e., forward bending) of thespine.

A “significant” increase in the medial-lateral range of motion refers toa difference in the range of motion that approximates the natural motionof the spine to a degree sufficient to be noticeable by the patient.More precisely, a “significant” increase may refer to the existence ofat least one additional millimeter of clearance between articulatingsurfaces of a facet joint under flexion, as compared to the same facetjoint under extension. Furthermore, a “significant” increase in themedial-lateral range of motion may refer to the existence of twoadditional millimeters of clearance between the articulating surfaces.

As shown in FIG. 53H, one of the articulating surfaces 1122, 1174, forexample, the articulating surface 1122 of the superior facet prosthesis1100, has a cephalad end 1250 and a caudal end 1252. The articulatingsurface 1122 also has a radius of curvature 1254 about an axis 1256extending generally from the cephalad end 1250 end to the caudal end1252. The radius of curvature 1254 changes along the axis 1256 toprovide greater clearance between the articulating surfaces 1122, 1174when the spine is under flexion. Similarly, the changing radius ofcurvature 1254 provides less clearance between the articulating surfaces1122, 1174 when the spine is extended.

In this embodiment, the articulating surface 1122 is shaped such that,when the superior facet prosthesis 1100 is coupled to the vertebra, theaxis 1256 is significantly anteriorly inclined at the cephalad end 1250to provide greater clearance between the articulating surfaces 1122,1174 when the spine is under flexion. In addition to or in thealternative to variation of the radius of curvature 1254 from thecephalad end 1250 to the caudal end 1252, the radius of curvature 1254could vary along a medial-lateral direction of the articulating surface.

More precisely, with brief reference to FIG. 51 again, the radius ofcurvature may be larger toward a medial end 1260 and a lateral end 1262of the articulating surface 1122 than at a central portion 1264 thereof.The radius of curvature could also be substantially infinite toward themedial and lateral ends, such that the articulating surface of thesuperior prosthesis has a curved region 1268 proximate the centralportion 1264, a first tangent flat 1270 disposed medially of and tangentto the curved region 1268, and a second tangent flat 1272 disposedlaterally of and tangent to the curved region 1268.

If desired, the inferior facet prosthesis may have an articulatingsurface with a three-dimensionally curved, generally elliptical shape. Athree-dimensionally curved, generally elliptical shape may have theappearance of a stretched spheroid or the like. Accordingly, athree-dimensionally curved, generally elliptical shape has a first crosssection having a generally elliptical shape and a second cross sectionperpendicular to the first cross section, having a semicircular shape.Alternatively, an inferior facet prosthesis may have an articulatingsurface with a generally cylindrical or semispherical shape, asillustrated in connection with FIGS. 40, 53B, and 53C, for example.

According to one alternative embodiment, the articulating surface of thesuperior facet prosthesis may have a uniform, substantially unchangingradius of curvature. The relative medial-lateral motion between thevertebra and the adjacent vertebra may still increase significantly withflexion of the spine due to the curvature of the inferior facetprosthesis. The radius of curvature of the articulating surface of theinferior facet prosthesis may change along an axis thereof, either alongthe cephalad-caudal direction or along the medial-lateral direction, toprovide greater clearance between the articulating surfaces when thespine is under flexion. According to yet another alternative, thevariation in motion in the medial-lateral direction may be obtained, notthrough a variable radius of curvature, but rather, through the relativepositioning of the superior and inferior facet prostheses.

Returning to FIGS. 53F, 53G, 53H, and 53I, the materials used toconstruct the articulating surfaces of the prostheses 1100, 1174 may beselected from a group consisting of a polymeric material, a polymericbearing material attached to a metal substrate, a ceramic bearingmaterial, a metal bearing material, and combinations thereof. A varietyof surface coatings, treatments, and the like may be used to enhance theperformance and/or longevity of the prostheses 1100, 1174.

The superior facet prosthesis 1100 may be shaped such that, when thesuperior facet prosthesis 1100 is coupled to the vertebra 102, the axis1256 is significantly anteriorly inclined from a longitudinal axis (notshown) of the spine to provide greater clearance between thearticulating surfaces 1122, 1174 when the spine is under flexion. Inthis application, “significantly anteriorly inclined” refers to thepresence of a deliberate offset, from the longitudinal axis of thespine, that has a meaningful effect on the facet joint of which thecorresponding prosthesis is a part. The offset angle 1259 between theaxis 1256 and the longitudinal axis 1258 of the spine may range fromabout −2.5° to about 14.5°. More precisely, the offset angle 1259 mayrange from about 5° to about 10°. Yet more precisely, the offset angle1259 may be about 7.25°.

Referring briefly again to FIG. 51 and FIG. 53G, one method of selectinginferior and superior facet prosthesis will be described. Theappropriate prosthesis of the kit of superior facet prostheses may beselected by, for example, forming a semicircular resection centered at aposition along the pedicle axis 1170 of the vertebra 102, at a knowndisplacement from the saddle point S1. Certain offsets, such as X₁ andX₂, as shown in FIG. 51, may be measured with between the saddle pointS1 and the most medial and anterior point P1.

Based on X₁ and X₂, values of the resection angle α and the facet angleβ may be obtained. The values of α and β may be used to select theappropriate superior facet prosthesis of the kit by, for example,looking up the values of α and β on lookup table or the like. Theremaining dimensions of the selected superior facet prosthesis may thusbe determined based on the combination of α and β.

The appropriate prosthesis of the kit of inferior facet prostheses mayalso be selected by making a limited number of measurements. Moreprecisely, a semicircular resection may be formed at a position centeredalong the pedicle axis of the vertebra 101, at a known displacement fromthe saddle point S2. One or more of the offsets IX, IY, and IZ may bemeasured between the resections of the saddle points S1 and S2.

Based on the values of IX, IY, and/or IZ obtained, the values of Iα andZ (as illustrated in FIG. 53C) are determined. The values of Iα and Zmay be used to select the appropriate inferior facet prosthesis of thekit by, for example, looking up the values of Iα and Z on a lookup tableor the like. The remaining dimensions of the selected inferior facetprosthesis may thus be determined based on the combination of Iα and Z.

The above-described selection method is beneficial because a relativelysmall number of linear measurements may be made to determine which setof prostheses is most appropriate for a given patient. Ease ofmeasurement is important because the measurements must generally beperformed during surgery. Accordingly, easier, more rapid measurementsenable surgery to be more rapidly and safely carried out. In alternativeembodiments, different measurement schemes may be carried out, and mayinclude different linear measurements, angular measurements, and thelike. In this application, measuring the “relative positions” of bonylandmarks may include measurement of linear displacements, angulardisplacements, or any combination thereof.

In alternative embodiments, a kit of superior and/or inferior prosthesisneed not have multiple one-piece prostheses, but may instead havemultiple components designed to be assembled together to provide aprosthesis having the necessary parameters. For example, each of aplurality of semispherical bone contacting portions may be connectableto any of a plurality of articulating surfaces, via a plurality ofconnecting members. Selecting a prosthesis may then entail selecting abone contacting portion, an articulating surface, and a connectingmember. The bone contacting portion, articulating surface, andconnecting member may then be coupled together via set screws,adhesives, interference fits, or the like.

If desired, the manner in which the various components are attachedtogether may also be adjustable to enable further adjustability of thedimensions of a selected prosthesis. Such a kit of components may alsoinclude additional components such as bearing surfaces, as described inconnection with FIG. 16. As yet another alternative, a single prosthesismay be adjustably deformed, for example, through the use of alever-operated manual press, a hydraulic press, or the like, to providethe desired dimensions prior to attachment to a patient's vertebra.

After a semispherical resection 1146 has been formed in a vertebra andthe corresponding prosthesis has been selected, a flat resection, suchas the first resection surface 1112 of FIG. 48, may be formed. The flatresection may be contiguous with the semispherical resection 1146, ormay be separated from the semispherical resection 1146 by an expanse ofunresected bone. The determination of which prosthesis to use may alsoindicate to the surgeon the proper placement of the flat resection toproperly receive the selected prosthesis. After the flat resection hasbeen formed, the selected prosthesis may be attached to the vertebra.The procedure may be the same as or similar to that described above forinstallation of the inferior and superior facet prostheses.

FIG. 54 is a dorsal view of a bilateral inferior facet prosthesis system1200 in situ. The bilateral inferior facet prosthesis system 1200 is amulti-piece inferior and superior facet prosthesis that has both a rightinferior facet prosthesis 1202 and a left inferior facet prosthesis 1204connected by a crosslink, which may take the form of a stabilizing bar1210. Both the right inferior facet prosthesis 1202 and the leftinferior facet prosthesis 1204 are designed to be affixed to the topvertebra 101 at the respective inferior facet resection surfaces 121(FIG. 19).

The bilateral inferior facet prostheses 1202, 1204 allow replacement ofboth the left and the right inferior facets. In this embodiment, theinferior prostheses are placed over left and right fixation elements1232, 1234 that extend into the top vertebra 101. In the embodimentshown in FIG. 54, the right inferior side is articulating against aright superior facet prosthesis 1100 attached to the first resectionsurface 1112 (FIG. 49) of the bottom vertebra 102. Also in thisembodiment, the left inferior facet prosthesis 1204 is articulatingagainst the left natural superior facet of the bottom vertebra 102.

The stabilizing bar 1210 of the bilateral inferior prosthesis system1200 is designed to stabilize the left inferior facet prosthesis 1204and the right inferior facet prosthesis 1202 so that they are secure.The stabilizing bar 1210 also allows the left and right inferior facetprostheses 1204, 1202 to support each other rather than requiringstabilizing members to be coupled to the spine lamina or the resectedinferior facet tissue. Further, the stabilizing bar 1210 can compressthe left and right inferior facet prostheses 1202, 1204 against theresected bone to improve bony ingrowth and apposition.

As also shown in FIG. 54, the stabilizing bar 1210 is coupled to theleft and right inferior prostheses 1202, 1204 by a gripping mechanism.The gripping mechanism may include any of a variety of structures,including clips, clamps, adhesive-bonds, threaded fasteners, and thelike. In the embodiment of FIG. 54, the gripping mechanism includes foreand aft flanges 1212, 1214 that engage the stabilizing bar 1210 to forma groove-and-rod joint. The fore and aft flanges 1212, 1214 arecompressed together with threaded turnbuckles 1216, 1218 to pinch thestabilizing bar 1210 there between.

The pinching action of the flanges 1212, 1214 allows the distancebetween the left and right inferior prostheses 1202, 1204 to be adjustedto best suit the anatomy of the patient. During surgery, the surgeonwould use a tool (not shown) to compress the left and right inferiorprostheses 1202, 1204 to the desired positions and then tighten theturnbuckles 1216, 1218 to secure the stabilizing bar 1210.

FIG. 55 is a perspective view of the bilateral inferior facet prosthesissystem 1200. The right inferior facet prosthesis 1202 includes a convexarticulating surface 1220 that engages an articulating surface 1122 ofthe superior facet prosthesis 1100. In one embodiment, the articulatingsurface 1122 of the superior facet prosthesis 1100 has a concave shape(FIGS. 47, 51).

In this application, the term “convex” relates to a surface that bulgesoutward with a three-dimensional curvature. Accordingly, a convexsurface is not just a sectorial portion of a cylinder, but rather, hassome outward curvature along two perpendicular directions. A convexsurface may be “semispherical,” or in other words, may include somesectorial portion of a sphere, which may be less than a hemisphere.However, a convex surface need not be semispherical, but may insteadhave contouring that provides a portion of an oval, elliptical,parabolic, and/or irregular cross sectional shape. A convex surface alsoneed not be curved in whole or in part, but may instead have one or moreplanar portions.

In this application, “concave” refers to a surface with a centralportion that is recessed with respect to at least two peripheralportions positioned on either side of the central portion. A concavesurface may be formed by planar regions, curves, or combinationsthereof. The central portion may be recessed along only one dimension,as with a surface defined by an interior section of a cylindrical wall.Alternatively, the central portion may be recessed along twoperpendicular dimensions, so that the central portion is recessed withrespect to at least four peripheral portions arranged around the centralportion. Accordingly, the surface may include a semispherical section, athree-dimensional parabolic or ellipsoidal section, or any otherthree-dimensionally curved shape.

As another alternative, the central portion of a concave surface may berecessed along one direction and distended with respect to aperpendicular direction, so that the concave surface takes on a shapesimilar to that of the rounded groove of a pulley that is designed toreceive a rope. Like a convex surface, a concave surface need not becurved in whole or in part, but may instead have one or more planarportions.

FIG. 56 is a lateral view of the bilateral inferior facet prosthesissystem 1200 and superior facet prosthesis 1100. The right inferiorprosthesis 1202 includes a member 1230 upon which the flanges 1212, 1214clamp. In the embodiment shown, the member 1230 is a ball-shaped member1230 upon which the flanges 1212, 1214 clamp to form a ball-and-socketjoint. The ball-and-socket joint and groove-and-rod joint providemultiple degrees of freedom for variable positioning of the left andright inferior prostheses 1202, 1204. More precisely, the ball- andsocket joint enables tri-axial rotation, i.e., rotation about threeperpendicular axes, until the flanges 1212, 1214 are pressed about themember 1230 to resist further relative rotation.

The ball-and-socket joint enables relative motion between the inferiorprostheses 1202, 1204 along the anterior/posterior directions and alongthe cephalad/caudal directions. The groove-and-rod joint enablesrelative motion between the inferior prostheses 1202, 1204 along thelateral/medial directions. However, when the turnbuckles 1216, 1218 aretightened, the displacement between the ball-shaped members 1230 of theinferior prostheses 1202, 1204 becomes fixed, and the ball-shapedmembers 1230 are no longer freely pivotable with respect to the flanges1212, 1214. Thus, the relative positions and orientations of theinferior prostheses 1202, 1204 may be fixed by tightening theturnbuckles 1216, 1218.

An alternative embodiment replaces the ball shaped member 1230 with amember (not shown) of differing shape and flanges adapted to engage thealternative member. Other potential shapes that allow a range ofadjustability and movement between the left and right inferiorprostheses 1202, 1204 and the flanges 1212, 1214 prior to clampinginclude, but are not limited to, columnar and annular shapes.

The ball-shaped member 1230 shown in FIG. 56 has several divots formedthereon. Upon compression of the flanges 1212, 1214, the flanges deforminto the divots to provide enhanced coupling and resistance to slippagetherebetween. An alternative embodiment of the ball-shaped member 1230has circumferential or axial splines (FIG. 63) formed thereon, which“bite” into the flanges 1212, 1214. Other alternative surface featuresof the ball-shaped member 1230 include knurling, nubs, grooves, facets,and combinations of any of the above.

Similarly, the stabilizing bar 1210 can have surface features to enhancecoupling to the flanges 1212, 1214. Exemplary surface features includelongitudinal splines, knurling, divots, nubs, and grooves. Splinesprevent rotation of the stabilizing bar 1210 with respect to the flanges1212, 1214. Knurling advantageously prevents both rotation andtranslation of the stabilizing bar 1210 with respect to the flanges1212, 1214.

The flanges 1212, 1214 can be formed of a material softer than that ofthe stabilizing bar 1210 and the ball-shaped members 1230 to furtherenhance coupling. Illustrative materials for the stabilizing bar 1210and ball-shaped members 1230 are Cobalt-Chrome (Co—Cr) alloys, Titanium(Ti) and stainless steel alloys. However, other biocompatible materialssuch as rigid polymers including PEEK and PEAK can be formed into theshapes of the stabilization bar 1210, and/or the ball-shaped members1230. In one alternative embodiment, the flanges 1212, 1214 are formedintegrally with or rigidly attached to the left and right inferiorprostheses 1202, 1204.

Referring again to FIG. 54, ends of the flanges 1212, 1214 that engagethe stabilizing bar 1210 are angled towards each other. This anglingavoids interference with surrounding bone and avoids interference withthe superior facet or the superior facet prosthesis 1100.

With continued reference to FIG. 54, it is seen that the heads of theturnbuckles 1216, 1218 can vary in size. As shown, the turnbuckle 1216is larger than the turnbuckle 1218. The larger head of the turnbuckle1216 allows the surgeon to exert more torque on the turnbuckle 1216,thereby allowing a more secure coupling of the flanges 1212, 1214 to thestabilizing bar 1210. The smaller head of the turnbuckle 1218 requiresless space at the surgical site of the patient than the larger head ofthe turnbuckle 1216. Therefore, the surgeon can select a turnbuckle headhaving the desired size, weighing the benefits of more applied torque ofthe larger head with the reduced spatial requirements of the smallerhead.

An alternative embodiment replaces the stabilizing bar 1210 with aflexible link, such as a cable of a biocompatible material. Yet anotheralternative embodiment includes a stabilizing bar having threaded ends.Instead of pinching flanges, the threaded ends of the stabilizing barextend through flanges of the left and right inferior prostheses 1202,1204. Threaded fasteners engage the threaded ends of the stabilizingbar. The threaded fasteners are then tightened to provide the desiredpositioning of the left and right inferior prostheses 1202, 1204. Inanother variation, the stabilizing bar is rotated such that the threadsof the stabilizing bar engage fixed threaded portions of the flanges.

FIG. 57 is a cranial view of the bilateral inferior facet prosthesissystem 1200.

FIG. 58 is a bottom in situ view of the bilateral inferior facetprosthesis system 1200 in situ.

FIG. 59 is rear view of the bilateral inferior facet prosthesis system1200 in isolation.

FIG. 60 is a top view of the bilateral inferior facet prosthesis system1200.

FIG. 61 is a bottom view of the bilateral inferior facet prosthesissystem 1200.

FIG. 62 is a perspective view of the right inferior prosthesis 1204.

FIGS. 63 and 64 are perspective and end views, respectively, of variousball-shaped members 1240, 1242, 1244 that may be included in theinferior prostheses 1202, 1204 in place of the members 1230, theball-shaped members 1240, 1242, 1244 having differing surface features,particularly circumferential grooves 1302, longitudinal grooves 1304,and knurling 1306.

FIG. 65 is a dorsal view of the bilateral inferior facet prosthesissystem 1200, in which castle nuts 1320 are attached to the left andright fixation elements 1232, 1234 and to the fixation member 1110.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notby way of limitation. Thus, the breadth and scope of the inventionshould not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. An implantable system comprising: an inferior prosthesis configuredto replicate a movement of an inferior facet joint, the inferiorprosthesis including a first articulating surface; a superior prosthesisconfigured to replicate a movement of a superior facet joint, thesuperior prosthesis including a second articulating surface that engagesthe first articulating surface; and a ball and socket joint forproviding the inferior prosthesis with motion in an anterior/posteriordirection.
 2. The system of claim 1, wherein the articulating surface ofthe inferior prosthesis is convex.
 3. The system of claim 1, wherein thearticulating surface of the superior prosthesis is concave.
 4. Thesystem of claim 1, further comprising turnbuckles that fix displacementof the ball and socket joint.
 5. The system of claim 1, wherein the balland socket joint comprises a ball-shaped member having divots formedthereon.
 6. The system of claim 1, wherein the first articulatingsurface is formed of a polymeric material.
 7. The system of claim 1,wherein the first articulating surface is formed of a polymeric materialattached to a metal substrate.
 8. An implantable system comprising: aleft inferior prosthesis and a right inferior prosthesis; and a crossbarattached to the left inferior prosthesis and the right inferiorprosthesis, wherein the left inferior prosthesis is coupled to thestabilizing bar by a first gripping mechanism and the right inferiorprosthesis is coupled to the stabilizing bar by a second grippingmechanism, the first and second gripping mechanisms comprising flangesthat form a groove-and-rod joint.
 9. The system of claim 8, furthercomprising a left superior prosthesis for engaging the left inferiorprosthesis and a right superior prosthesis for engaging the rightinferior prosthesis.
 10. The system of claim 8, wherein the flanges arecompressed with turnbuckles to pinch the stabilizing bar therebetween.11. The system of claim 10, wherein the distance between the leftinferior prosthesis and right inferior prosthesis can be adjusted alongthe length of the crossbar.
 12. The system of claim 8, wherein theflanges further clamp a ball-shaped member to form ball-and-socketjoints.
 13. An implantable system comprising: a left inferior prosthesisand a right inferior prosthesis; and a crossbar attached to the leftinferior prosthesis and the right inferior prosthesis, wherein the leftinferior prosthesis is attachable to a first portion of the crossbar viaa first gripping mechanism and the right inferior prosthesis isattachable to a second portion of the crossbar via a second grippingmechanism, wherein the distance between the left inferior prosthesis andthe right inferior prosthesis is adjustable.
 14. The system of claim 13,wherein the first gripping mechanism comprises a pair of flanges. 15.The system of claim 14, further comprising a threaded turnbuckle forcompressing together the pair of flanges.
 16. The system of claim 13,wherein the left inferior prosthesis comprises an articulating bearingsurface.
 17. The system of claim 16, wherein the articulating bearingsurface is convex.
 18. The system of claim 13, further comprising a leftsuperior surface for engaging the left inferior surface.
 19. The systemof claim 18, wherein the left superior surface comprises a concavearticulating bearing surface.
 20. The system of claim 13, wherein thecrossbar is formed of cobalt-chrome.